Building with gcc toolchain and C++ API produces lots of warnings

[lutter]

When I try to build mruby and use conf.enable_cxx_abi, the compiler emits lots of warnings of the form

CC    src/range.c -> build/host/src/range.o
cc1plus: warning: command line option ‘-Wdeclaration-after-statement’ is valid for C/ObjC but not for C++
cc1plus: warning: command line option ‘-std=gnu99’ is valid for C/ObjC but not for C++

The problem seems to be that enable_cxx_abi simply appends the cxx_compile_flag options to the normal compiler options resulting in conflicting compiler flags. I am not quite sure what the best way to fix this would be.

It also seems that with the way enable_cxx_abi is currently done, it makes it impossible for mrbgems to compile anything as plain C. It looks like that is a problem e.g. for mruby-onig-regexp which needs to compile the bundled onigmo as C, not C++, even if C++ ABI is enabled.

I am on Fedora 25 with gcc 6.3.1.

[matz]

I understand the issue but have no idea to fix it. Besides that, it's an only warning issue with low priority to fix.

[lutter]

Thanks for the quick reply - I agree that it's mostly cosmetic at this point. If I'll figure something out, I'll post something here.