Support custom PKG_CONFIG_PATH in the wasm build

[rchiodo]

Feature or enhancement

tools/wasm/wasm_build.py does not have an easy way to build submodules.

Pitch

PKG_CONFIG_PATH could be used to support say things like https://github.com/hoodmane/libffi-emscripten.

I propose adding this to the wasm_build.py to support custom package configs.

Previous discussion

https://discuss.python.org/t/get-ctypes-working-in-emscripten-build/18466/2

[tiran]

Actually there is no need to change any code in CPython. It turns out that Emscripten has an undocumented env var EM_PKG_CONFIG_PATH. I have opened emscripten-core/emscripten#17736 to add the env var to Emscripten docs.

[tiran]

@rchiodo Can you confirm that EM_PKG_CONFIG_PATH solves your problem?

[rchiodo]

Yes that did work for me. Thanks.