Change -Wunused-but-set-variable to GCC only

[mendsley]

Clang (as of version 10.0.0) does not support this flag.

root@1cbb45e200ad:/work/bimg# ../genie/bin/linux/genie --gcc=linux-clang gmake
Building configurations...
Running action 'gmake'...
Done. Generated 0/5 projects.

root@1cbb45e200ad:/work/bimg# make config=debug64 -C .build/projects/gmake-linux-clang -f bimg_decode.make clean all
make: Entering directory '/work/bimg/.build/projects/gmake-linux-clang'
Cleaning bimg_decode
Creating ../../linux64_clang/obj/x64/Debug/bimg_decode
Creating ../../linux64_clang/obj/x64/Debug/bimg_decode/src
image_decode.cpp
../../../src/image_decode.cpp:60:1: warning: unknown warning group '-Wunused-but-set-variable', ignored [-Wunknown-warning-option]
BX_PRAGMA_DIAGNOSTIC_IGNORED_CLANG_GCC("-Wunused-but-set-variable");
^
../../../../bx/include/bx/macros.h:193:49: note: expanded from macro 'BX_PRAGMA_DIAGNOSTIC_IGNORED_CLANG_GCC'
#       define BX_PRAGMA_DIAGNOSTIC_IGNORED_CLANG_GCC BX_PRAGMA_DIAGNOSTIC_IGNORED_CLANG
                                                      ^
../../../../bx/include/bx/macros.h:163:49: note: expanded from macro 'BX_PRAGMA_DIAGNOSTIC_IGNORED_CLANG'
#       define BX_PRAGMA_DIAGNOSTIC_IGNORED_CLANG(_x) _Pragma(BX_STRINGIZE(clang diagnostic ignored _x) )
                                                      ^
<scratch space>:6:27: note: expanded from here
 clang diagnostic ignored "-Wunused-but-set-variable"
                          ^
1 warning generated.
Archiving bimg_decode
make: Leaving directory '/work/bimg/.build/projects/gmake-linux-clang'

root@1cbb45e200ad:/work/bimg# clang --version
clang version 10.0.0-4ubuntu1
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin