`subxt-signer` with feature "ecdsa" does not compile to `no-std` right now.

If the ecdsa features is enabled in subxt-signer which pulls in the secp256k1 crate, there is some C source code that does not want to compile via cargo build --target thumbv7em-none-eabi

error: failed to run custom build command for `secp256k1-sys v0.9.2`

Caused by:
  process didn't exit successfully: `/home/tadeo/code/subxt/testing/no-std-tests/target/debug/build/secp256k1-sys-6ae4a38afd356447/build-script-build` (exit status: 1)
  --- stdout
  TARGET = Some("thumbv7em-none-eabi")
  OPT_LEVEL = Some("0")
  HOST = Some("x86_64-unknown-linux-gnu")
  cargo:rerun-if-env-changed=CC_thumbv7em-none-eabi
  CC_thumbv7em-none-eabi = None
  cargo:rerun-if-env-changed=CC_thumbv7em_none_eabi
  CC_thumbv7em_none_eabi = None
  cargo:rerun-if-env-changed=TARGET_CC
  TARGET_CC = None
  cargo:rerun-if-env-changed=CC
  CC = None
  RUSTC_LINKER = None
  cargo:rerun-if-env-changed=CROSS_COMPILE
  CROSS_COMPILE = None
  cargo:rerun-if-env-changed=CC_ENABLE_DEBUG_OUTPUT
  cargo:warning=Compiler version doesn't include clang or GCC: "arm-none-eabi-gcc" "--version"
  cargo:rerun-if-env-changed=CRATE_CC_NO_DEFAULTS
  CRATE_CC_NO_DEFAULTS = None
  DEBUG = Some("true")
  CARGO_CFG_TARGET_FEATURE = Some("dsp,mclass,thumb-mode,thumb2,v5te,v6,v6k,v6t2,v7")
  cargo:rerun-if-env-changed=CFLAGS_thumbv7em-none-eabi
  CFLAGS_thumbv7em-none-eabi = None
  cargo:rerun-if-env-changed=CFLAGS_thumbv7em_none_eabi
  CFLAGS_thumbv7em_none_eabi = None
  cargo:rerun-if-env-changed=TARGET_CFLAGS
  TARGET_CFLAGS = None
  cargo:rerun-if-env-changed=CFLAGS
  CFLAGS = None
  cargo:rerun-if-env-changed=CC_thumbv7em-none-eabi
  CC_thumbv7em-none-eabi = None
  cargo:rerun-if-env-changed=CC_thumbv7em_none_eabi
  CC_thumbv7em_none_eabi = None
  cargo:rerun-if-env-changed=TARGET_CC
  TARGET_CC = None
  cargo:rerun-if-env-changed=CC
  CC = None
  RUSTC_LINKER = None
  cargo:rerun-if-env-changed=CROSS_COMPILE
  CROSS_COMPILE = None
  cargo:rerun-if-env-changed=CC_ENABLE_DEBUG_OUTPUT
  cargo:warning=Compiler version doesn't include clang or GCC: "arm-none-eabi-gcc" "--version"
  cargo:rerun-if-env-changed=CRATE_CC_NO_DEFAULTS
  CRATE_CC_NO_DEFAULTS = None
  CARGO_CFG_TARGET_FEATURE = Some("dsp,mclass,thumb-mode,thumb2,v5te,v6,v6k,v6t2,v7")
  cargo:rerun-if-env-changed=CFLAGS_thumbv7em-none-eabi
  CFLAGS_thumbv7em-none-eabi = None
  cargo:rerun-if-env-changed=CFLAGS_thumbv7em_none_eabi
  CFLAGS_thumbv7em_none_eabi = None
  cargo:rerun-if-env-changed=TARGET_CFLAGS
  TARGET_CFLAGS = None
  cargo:rerun-if-env-changed=CFLAGS
  CFLAGS = None
  cargo:rerun-if-env-changed=CC_thumbv7em-none-eabi
  CC_thumbv7em-none-eabi = None
  cargo:rerun-if-env-changed=CC_thumbv7em_none_eabi
  CC_thumbv7em_none_eabi = None
  cargo:rerun-if-env-changed=TARGET_CC
  TARGET_CC = None
  cargo:rerun-if-env-changed=CC
  CC = None
  RUSTC_LINKER = None
  cargo:rerun-if-env-changed=CROSS_COMPILE
  CROSS_COMPILE = None
  cargo:rerun-if-env-changed=CC_ENABLE_DEBUG_OUTPUT
  cargo:warning=Compiler version doesn't include clang or GCC: "arm-none-eabi-gcc" "--version"
  cargo:rerun-if-env-changed=CRATE_CC_NO_DEFAULTS
  CRATE_CC_NO_DEFAULTS = None
  CARGO_CFG_TARGET_FEATURE = Some("dsp,mclass,thumb-mode,thumb2,v5te,v6,v6k,v6t2,v7")
  cargo:rerun-if-env-changed=CFLAGS_thumbv7em-none-eabi
  CFLAGS_thumbv7em-none-eabi = None
  cargo:rerun-if-env-changed=CFLAGS_thumbv7em_none_eabi
  CFLAGS_thumbv7em_none_eabi = None
  cargo:rerun-if-env-changed=TARGET_CFLAGS
  TARGET_CFLAGS = None
  cargo:rerun-if-env-changed=CFLAGS
  CFLAGS = None
  cargo:warning=depend/secp256k1/contrib/lax_der_parsing.c:7:10: fatal error: string.h: No such file or directory
  cargo:warning=    7 | #include <string.h>
  cargo:warning=      |          ^~~~~~~~~~
  cargo:warning=compilation terminated.
  cargo:warning=In file included from depend/secp256k1/src/util.h:13,
  cargo:warning=                 from depend/secp256k1/src/field.h:10,
  cargo:warning=                 from depend/secp256k1/src/group.h:10,
  cargo:warning=                 from depend/secp256k1/src/precomputed_ecmult_gen.c:3:
  cargo:warning=/usr/lib/gcc/arm-none-eabi/13.2.0/include/stdint.h:9:16: fatal error: stdint.h: No such file or directory
  cargo:warning=    9 | # include_next <stdint.h>
  cargo:warning=      |                ^~~~~~~~~~
  cargo:warning=compilation terminated.

  --- stderr


  error occurred: Command "arm-none-eabi-gcc" "-O0" "-ffunction-sections" "-fdata-sections" "-g" "-fno-omit-frame-pointer" "-mthumb" "-march=armv7e-m" "-I" "depend/secp256k1/" "-I" "depend/secp256k1/include" "-I" "depend/secp256k1/src" "-I" "wasm/wasm-sysroot" "-Wall" "-Wextra" "-Wno-unused-function" "-Wno-unused-parameter" "-DSECP256K1_API=" "-DENABLE_MODULE_ECDH=1" "-DENABLE_MODULE_SCHNORRSIG=1" "-DENABLE_MODULE_EXTRAKEYS=1" "-DENABLE_MODULE_ELLSWIFT=1" "-Dprintf(...)=" "-DECMULT_GEN_PREC_BITS=4" "-DECMULT_WINDOW_SIZE=15" "-DUSE_EXTERNAL_DEFAULT_CALLBACKS=1" "-DENABLE_MODULE_RECOVERY=1" "-o" "/home/tadeo/code/subxt/testing/no-std-tests/target/thumbv7em-none-eabi/debug/build/secp256k1-sys-74715f18bcbdb71a/out/c928380082b86401-precomputed_ecmult_gen.o" "-c" "depend/secp256k1/src/precomputed_ecmult_gen.c" with args "arm-none-eabi-gcc" did not execute successfully (status code exit status: 1).

Start in testing/no-std-tests and reproduce the issue with cargo build --target thumbv7em-none-eabi while enabling subxt-signer's ecdsa feature. Inspect the secp256k1-sys build output and dependency path shown in the failure. Done means the no-std target builds successfully with ecdsa enabled.