[FEATURE] Test that headers can compile in isolation. about bpt HOT 3 CLOSED

I did some investigation into implementing this and ran into an issue with gcc: gcc warns on #pragma once in "main file", and AFAICT, the warning can't be turned off ( https://godbolt.org/z/TKMfMoP7h ) (well, -w does it, but it also turns off all warnings).

Clang has a way to turn off the warning, and MSVC doesn't warn.

I suppose this means that the general way of implementing this would be to generate a .cpp file with just a #include "the-header-file" and compile that with -fsyntax-only. Unless we want to make a way to suppress warnings that the compiler doesn't support suppression of.

For reference, a GCC bug report on the inability to turn off the warning for #pragma once in non-header files: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89808 . Too bad that won't help us when it's fixed, as we'll want to support older versions of gcc.

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I suppose we could pipe in the #include "the-header-file" to gcc and avoid the need to generate an actual file.

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Added in #78

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