General questions regarding OpenSSL linking

Im struggeling with compiling OpenSSL for emscripten. Also there are a few things that i dont really understand.

I tried to compile OpenSSL 1.1.1u on ubuntu.
The only special flag added to the config is the no-tests, since some tests had duplicate symbols for me, so i just didnt compile them at all.
`emconfigure ./Configure no-tests linux-generic64 --prefix=$EMSCRIPTEN/system`

Next i executed this command to change the cross compilation directory? Actually i have no idea why and i cant tell anymore where i did get this from, but without this the compilation fails:
`sed -i 's|^CROSS_COMPILE.*$|CROSS_COMPILE=|g' Makefile`

Then i modified the Makefile manually to compile OpenSSL with emscripten pthreads because my other code uses pthreads and also to compile OpenSSL as a sidemodule. I think this is necessary so that dlopen() works which is needed by OpenSSL, but please correct me if im wrong. (Im not sure if there is a way to compile OpenSSL so that it does not need dlopen() to work)
`CFLAGS=-O3 -Wall -s USE_PTHREADS -sSIDE_MODULE`

Finally i just start the compilation:
`emmake make`

I put the generated *.a files (libcrypto.a and libssl.a) into the root directory of my project.

The project itself im compiling with the following flags:
`em++ -sMAIN_MODULE -lembind -sASSERTIONS -std=c++17 -O0 -lwebsocket.js -pthread -sPROXY_TO_PTHREAD -sASYNCIFY -s ALLOW_MEMORY_GROWTH=1 ../Test.cpp -o out.html --shell-file index.html -I ../Libs/openssl_emscripten/include libcrypto.a libssl.a --bind`

I think the important parts here are the pthread linking and that libcrypto and libssl are listed here so that they can get linked. Also this is compiled as the MAIN_MODULE so that it can load the SIDE_MODULEs such as OpenSSL i suppose?
This produces this output:
```em++: warning: EXPORTED_FUNCTIONS is not valid with LINKABLE set (normally due to SIDE_MODULE=1/MAIN_MODULE=1) since all functions are exported this mode. To export only a subset use SIDE_MODULE=2/MAIN_MODULE=2 [-Wunused-command-line-argument]
em++: warning: -sMAIN_MODULE + pthreads is experimental [-Wexperimental]
em++: warning: -pthread + ALLOW_MEMORY_GROWTH may run non-wasm code slowly, see https://github.com/WebAssembly/design/issues/1271 [-Wpthreads-mem-growth]
error: undefined symbol: _Z13base64_decodeRKNSt3__212basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEE (referenced by top-level compiled C/C++ code)
warning: To disable errors for undefined symbols use `-sERROR_ON_UNDEFINED_SYMBOLS=0`
warning: __Z13base64_decodeRKNSt3__212basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEE may need to be added to EXPORTED_FUNCTIONS if it arrives from a system library
error: undefined symbol: _Z13base64_encodePKhj (referenced by top-level compiled C/C++ code)
warning: __Z13base64_encodePKhj may need to be added to EXPORTED_FUNCTIONS if it arrives from a system library
Error: Aborting compilation due to previous errors
em++: error: '/node/15.14.0_64bit/bin/node.exe E:\DATA\Libs\emscripten\emsdk-main\upstream\emscripten\src\compiler.js \AppData\Local\Temp\tmpzc6xe8_z.json' failed (returned 1)
```

I really dont understand this error message. It seems that some base64 encoding function is not present?