clang on OSX

clang on OSX

The Unix Makefile.{src,utl} use the compiler name 'gcc' by default on
OSX, and that invokes clang which defines __GNUC__ to deal with gcc
extensions.  But when invoked via the Xcode IDE, it evidently uses its
own name instead, and wasn't defining __GNUC__.  tradstdc.h started
down the road of duplicating gcc features; switch to pretending to be
gcc instead.