Eliminate reliance on special CLANG_GNUC_* macros.

The BSL platform header relied on custom preprocessor macros that had
to be provided by the build system when Clang was used to compile. In fact
these macros don't appear to be necessary, and the same effect can be
achieved by using the GNUC_* macros that Clang already defines as part
of its emulation of GCC.

This has been tested on OSX with Xcode 4.5, and on openSUSE 12.2 using both Clang 3.1 and GCC 4.7, and on the Linux matrix build of seven platforms.