Fix build with GCC:

 - GCC doesn't support #pragma mark, only Apple GCC and clang. Wrap the pragma mark's in #if 0 to make gcc ignore them but xcode still see them.
 - Wrap a bunch of "#pragma clang" lines in #ifdef __clang__.
 - Pacify gcc's -Wparenthesis in a case where it's quite reasonable.

llvm-svn: 231344