1) Remove cmake-specific #define usage from the exported

Block.h/Block_private.h headers, since clients won't know what to
set. These are moved into runtime.c as appropriate

2) Use cmake checks for CAS builtins, instead of guessing based on GCC
#defines (which aren't set by clang and llvm-gcc anyway)

3) "#pragma mark" isn't supported by FSF gcc, so "#if 0" it out. It
should still show up in IDEs that support it

4) Fix some compiler warnings. GCC 4.3.3 seems super strict about
%p. function pointers can't be cast to void * either.

5) Avoid a warning for apple_versioning.c that "ISO C does not allow
empty files"

llvm-svn: 82504