Add -Wno-error=attributes if the current compiler is GCC

[pavel-kirienko]

Because older versions of GCC do not recognize [[unused]] and hence break the build.

[serges147]

@pavel-kirienko Please give me details how to reproduce it. F.e. what exact "older" version of GCC I should try on? Thanks!
BTW, do we have somewhere documented which toolsets and their minimal versions we support?

[pavel-kirienko]

I was trying to build it on my laptop last night which has GCC 11, which produced this warning. This is a known issue; in another codebase we have this:

# Disable GCC warnings that produce false positives due to bugs in the compiler.
if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-unknown-attributes -Wno-maybe-uninitialized -Wno-uninitialized")
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-error=stringop-overread")
endif ()

Clang appears to be much more stable btw.