Conflicting #define noticed during build

[danielhams]

Not high priority, but would be nice to re-assure ourselves this isn't causing issues -> consistency

/builds/dan/bootstrapbuilds/gbs8_1-gcc8/build-gcc/mips-sgi-irix6.5/libstdc++-v3/include/mips-sgi-irix6.5/bits/c++config.h:1596: warning: "_GLIBCXX_USE_WCHAR_T" redefined
 #define _GLIBCXX_USE_WCHAR_T 1
 
In file included from /builds/dan/bootstrapbuilds/gbs8_1-gcc8/build-gcc/mips-sgi-irix6.5/libstdc++-v3/include/mips-sgi-irix6.5/bits/c++config.h:508,
                 from /builds/dan/bootstrapbuilds/gbs8_1-gcc8/gcc-8.2.0/libstdc++-v3/src/c++98/mt_allocator.cc:29:
/builds/dan/bootstrapbuilds/gbs8_1-gcc8/build-gcc/mips-sgi-irix6.5/libstdc++-v3/include/mips-sgi-irix6.5/bits/os_defines.h:40: note: this is the location of the previous definition
 #define _GLIBCXX_USE_WCHAR_T 0```