Guard GCC pragmas

[nlohmann]

This PR adds preprocessor checks to only use GCC pragmas with compatible compilers.

Fixes #2924

[coveralls]

Coverage remained the same at 100.0% when pulling e20f3f9 on issue2924 into 57d31d1 on develop.

[MHebes]

Just curious—there are other #pragma GCC s which are guarded on the __clang__ variable instead of __GNUC__. I think clang defines the latter, but gcc might not define the former. Would it make sense to change those to __GNUC__ as well?

[nlohmann]

Just curious—there are other #pragma GCC s which are guarded on the __clang__ variable instead of __GNUC__. I think clang defines the latter, but gcc might not define the former. Would it make sense to change those to __GNUC__ as well?

The only other place is this:

// disable documentation warnings on clang
#if defined(__clang__)
    #pragma GCC diagnostic push
    #pragma GCC diagnostic ignored "-Wdocumentation"
#endif

This pragma is meant for Clang, because only Clang has a -Wdocumentation warning. If we would change it to __GNUC__, then GCC would be confused about this unknown flag.

[t-b]

@nlohmann What is missing to avoid things like this with the CI? warning-free tests on all platforms?

[nlohmann]

@nlohmann What is missing to avoid things like this with the CI? warning-free tests on all platforms?

I currently set no specific warning flag for MSVC. Is there something comparable to -Werror which is still pragmatic for MSVC?

[t-b]

/WX for warnings as errors and /W4 for maximum warnings. Maybe you can scavenge something from 0b8644c.

[nlohmann]

I'll give it a try, see #2930 - feedback more than welcome!