Fails to compile when using strdup C23 standard function

Please include the following in your bug report:

**Version of emscripten/emsdk:**
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.74 (1092ec30a3fb1d46b1782ff1b4db5094d3d06ae5)
clang version 20.0.0git (https:/github.com/llvm/llvm-project 322eb1a92e6d4266184060346616fa0dbe39e731)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /Users/x/emsdk/upstream/bin

**Failing command line in full:**
Command line:
```
-> emcc src/usdc.c src/token.c external/lz4/lib/lz4.c -I external/lz4/lib -std=c23 -o build/wasm/wasm_usdc.html -Wall -Wextra --preload-file data -sMEMORY64=2
```
Output:
```
src/token.c:169:10: error: use of undeclared identifier 'strdup'; did you mean 'strupr'?
169 | .str = strdup(str),
| ^~~~~~
| strupr
/Users/urjasvisuthar/emsdk/upstream/emscripten/cache/sysroot/include/compat/string.h:9:14: note: 'strupr' declared here
9 | extern char* strupr(char *);
| ^
1 error generated.
emcc: error: '/Users/urjasvisuthar/emsdk/upstream/bin/clang -target wasm64-unknown-emscripten -fignore-exceptions -mno-bulk-memory -mno-bulk-memory-opt -mno-nontrapping-fptoint -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr --sysroot=/Users/urjasvisuthar/emsdk/upstream/emscripten/cache/sysroot -DEMSCRIPTEN -Werror=implicit-function-declaration -Xclang -iwithsysroot/include/fakesdl -Xclang -iwithsysroot/include/compat -Iexternal/lz4/lib -std=c23 -Wall -Wextra src/token.c -c -o /var/folders/7x/khcwdw3d39g8c3k03mcrcw3r0000gn/T/emscripten_temp_5hhhrejl/token_1.o' failed (returned 1)
```
---
**Full link command and output with `-v` appended:**
Command line:
```
emcc src/usdc.c src/token.c external/lz4/lib/lz4.c -I external/lz4/lib -std=c23 -o build/wasm/wasm_usdc.html -Wall -Wextra --preload-file data -sMEMORY64=2 -v
```
Output:
```
/Users/urjasvisuthar/emsdk/upstream/bin/clang -target wasm64-unknown-emscripten -fignore-exceptions -mno-bulk-memory -mno-bulk-memory-opt -mno-nontrapping-fptoint -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr --sysroot=/Users/urjasvisuthar/emsdk/upstream/emscripten/cache/sysroot -DEMSCRIPTEN -Werror=implicit-function-declaration -Xclang -iwithsysroot/include/fakesdl -Xclang -iwithsysroot/include/compat -Iexternal/lz4/lib -std=c23 -Wall -Wextra -v src/usdc.c -c -o /var/folders/7x/khcwdw3d39g8c3k03mcrcw3r0000gn/T/emscripten_temp_s3__nidp/usdc_0.o
clang version 20.0.0git (https:/github.com/llvm/llvm-project 322eb1a92e6d4266184060346616fa0dbe39e731)
Target: wasm64-unknown-emscripten
Thread model: posix
InstalledDir: /Users/urjasvisuthar/emsdk/upstream/bin
(in-process)
"/Users/urjasvisuthar/emsdk/upstream/bin/clang-20" -cc1 -triple wasm64-unknown-emscripten -emit-obj -disable-free -clear-ast-before-backend -disable-llvm-verifier -discard-value-names -main-file-name usdc.c -mrelocation-model static -mframe-pointer=none -ffp-contract=on -fno-rounding-math -mconstructor-aliases -target-cpu generic -target-feature -bulk-memory -target-feature -bulk-memory-opt -target-feature -nontrapping-fptoint -fvisibility=hidden -debugger-tuning=gdb -fdebug-compilation-dir=/Users/urjasvisuthar/Desktop/test/fta -v -fcoverage-compilation-dir=/Users/urjasvisuthar/Desktop/test/fta -resource-dir /Users/urjasvisuthar/emsdk/upstream/lib/clang/20 -D EMSCRIPTEN -I external/lz4/lib -isysroot /Users/urjasvisuthar/emsdk/upstream/emscripten/cache/sysroot -internal-isystem /Users/urjasvisuthar/emsdk/upstream/lib/clang/20/include -internal-isystem /Users/urjasvisuthar/emsdk/upstream/emscripten/cache/sysroot/include/wasm64-emscripten -internal-isystem /Users/urjasvisuthar/emsdk/upstream/emscripten/cache/sysroot/include -Werror=implicit-function-declaration -Wall -Wextra -std=c23 -ferror-limit 19 -fgnuc-version=4.2.1 -fskip-odr-check-in-gmf -fignore-exceptions -fcolor-diagnostics -iwithsysroot/include/fakesdl -iwithsysroot/include/compat -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr -o /var/folders/7x/khcwdw3d39g8c3k03mcrcw3r0000gn/T/emscripten_temp_s3__nidp/usdc_0.o -x c src/usdc.c
clang -cc1 version 20.0.0git based upon LLVM 20.0.0git default target x86_64-apple-darwin24.1.0
ignoring nonexistent directory "/Users/urjasvisuthar/emsdk/upstream/emscripten/cache/sysroot/include/wasm64-emscripten"
#include "..." search starts here:
#include <...> search starts here:
external/lz4/lib
/Users/urjasvisuthar/emsdk/upstream/emscripten/cache/sysroot/include/fakesdl
/Users/urjasvisuthar/emsdk/upstream/emscripten/cache/sysroot/include/compat
/Users/urjasvisuthar/emsdk/upstream/lib/clang/20/include
/Users/urjasvisuthar/emsdk/upstream/emscripten/cache/sysroot/include
End of search list.
/Users/urjasvisuthar/emsdk/upstream/bin/clang -target wasm64-unknown-emscripten -fignore-exceptions -mno-bulk-memory -mno-bulk-memory-opt -mno-nontrapping-fptoint -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr --sysroot=/Users/urjasvisuthar/emsdk/upstream/emscripten/cache/sysroot -DEMSCRIPTEN -Werror=implicit-function-declaration -Xclang -iwithsysroot/include/fakesdl -Xclang -iwithsysroot/include/compat -Iexternal/lz4/lib -std=c23 -Wall -Wextra -v src/token.c -c -o /var/folders/7x/khcwdw3d39g8c3k03mcrcw3r0000gn/T/emscripten_temp_s3__nidp/token_1.o
clang version 20.0.0git (https:/github.com/llvm/llvm-project 322eb1a92e6d4266184060346616fa0dbe39e731)
Target: wasm64-unknown-emscripten
Thread model: posix
InstalledDir: /Users/urjasvisuthar/emsdk/upstream/bin
(in-process)
"/Users/urjasvisuthar/emsdk/upstream/bin/clang-20" -cc1 -triple wasm64-unknown-emscripten -emit-obj -disable-free -clear-ast-before-backend -disable-llvm-verifier -discard-value-names -main-file-name token.c -mrelocation-model static -mframe-pointer=none -ffp-contract=on -fno-rounding-math -mconstructor-aliases -target-cpu generic -target-feature -bulk-memory -target-feature -bulk-memory-opt -target-feature -nontrapping-fptoint -fvisibility=hidden -debugger-tuning=gdb -fdebug-compilation-dir=/Users/urjasvisuthar/Desktop/test/fta -v -fcoverage-compilation-dir=/Users/urjasvisuthar/Desktop/test/fta -resource-dir /Users/urjasvisuthar/emsdk/upstream/lib/clang/20 -D EMSCRIPTEN -I external/lz4/lib -isysroot /Users/urjasvisuthar/emsdk/upstream/emscripten/cache/sysroot -internal-isystem /Users/urjasvisuthar/emsdk/upstream/lib/clang/20/include -internal-isystem /Users/urjasvisuthar/emsdk/upstream/emscripten/cache/sysroot/include/wasm64-emscripten -internal-isystem /Users/urjasvisuthar/emsdk/upstream/emscripten/cache/sysroot/include -Werror=implicit-function-declaration -Wall -Wextra -std=c23 -ferror-limit 19 -fgnuc-version=4.2.1 -fskip-odr-check-in-gmf -fignore-exceptions -fcolor-diagnostics -iwithsysroot/include/fakesdl -iwithsysroot/include/compat -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr -o /var/folders/7x/khcwdw3d39g8c3k03mcrcw3r0000gn/T/emscripten_temp_s3__nidp/token_1.o -x c src/token.c
clang -cc1 version 20.0.0git based upon LLVM 20.0.0git default target x86_64-apple-darwin24.1.0
ignoring nonexistent directory "/Users/urjasvisuthar/emsdk/upstream/emscripten/cache/sysroot/include/wasm64-emscripten"
#include "..." search starts here:
#include <...> search starts here:
external/lz4/lib
/Users/urjasvisuthar/emsdk/upstream/emscripten/cache/sysroot/include/fakesdl
/Users/urjasvisuthar/emsdk/upstream/emscripten/cache/sysroot/include/compat
/Users/urjasvisuthar/emsdk/upstream/lib/clang/20/include
/Users/urjasvisuthar/emsdk/upstream/emscripten/cache/sysroot/include
End of search list.
src/token.c:169:10: error: use of undeclared identifier 'strdup'; did you mean 'strupr'?
169 | .str = strdup(str),
| ^~~~~~
| strupr
/Users/urjasvisuthar/emsdk/upstream/emscripten/cache/sysroot/include/compat/string.h:9:14: note: 'strupr' declared here
9 | extern char* strupr(char *);
| ^
1 error generated.
emcc: error: '/Users/urjasvisuthar/emsdk/upstream/bin/clang -target wasm64-unknown-emscripten -fignore-exceptions -mno-bulk-memory -mno-bulk-memory-opt -mno-nontrapping-fptoint -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr --sysroot=/Users/urjasvisuthar/emsdk/upstream/emscripten/cache/sysroot -DEMSCRIPTEN -Werror=implicit-function-declaration -Xclang -iwithsysroot/include/fakesdl -Xclang -iwithsysroot/include/compat -Iexternal/lz4/lib -std=c23 -Wall -Wextra -v src/token.c -c -o /var/folders/7x/khcwdw3d39g8c3k03mcrcw3r0000gn/T/emscripten_temp_s3__nidp/token_1.o' failed (returned 1)
```
---

Google group conversation:
https://groups.google.com/g/emscripten-discuss/c/3DMpT62eLuE