GCC_PREFIX_HEADER & GCC_PRECOMPILE_PREFIX_HEADER incompatible with Electron 20

https://github.com/electron/rebuild/blob/29365ad904a8713567c4a7beb745d32df7f229e1/src/clang-fetcher.ts#L46

Hi --

I believe (not 100% sure) that this line of code is to blame. PCH (precompiled headers) need to be generated for every language in the project (c, c++, etc). I have a project that has a couple c files and is mostly c++. But the precompiled header is being compiled with -std=c++17 even for the c version of the header, which errors.

I'm not sure what the best solution is.

This issue arises on Mac as GCC_PREFIX_HEADER is part of XCODE_SETTINGS