fix RB_GC_GUARD_PTR on non-GCC/non-MSC builds

[lamont-granquist]

• the definition of RB_GC_GUARD_PTR needs to be moved into the block
  which is MSC-specific since in the non-MSC/non-GCC codepath here we
  do not have any rb_gc_guarded_ptr defined.
• i alised RB_GC_GUARD_PTR to RB_GC_GUARD which i think is how people
  are intending to use this macro?
• people are in fact using this macro as a public API even though
  there is the comment in the code not to.

[lamont-granquist]

See ffi/ffi#489

On AIX using ruby compiled with xlc and trying to use the ffi gem (which uses RB_GUARD_PTR) i get:

exec(): 0509-036 Cannot load program ruby because of the following errors:
rtld: 0712-001 Symbol rb_gc_guarded_ptr was referenced
      from module /opt/chef/embedded/lib/ruby/gems/2.2.0/gems/ffi-1.9.10/lib/ffi_c.so(), but a runtime definition
      of the symbol was not found.
rtld: 0712-002 fatal error: exiting.

The error is accurate because in non-GGC/non-Windows codepaths rb_gc_guarded_ptr is referenced from the RB_GC_GUARD_PTR macro but never defined.

At least the RB_GC_GUARD_PTR macro should be itself undefined to move this error to compile time rather than link type and make it a bit more obvious.

[nobu]

ffi/ffi#490 is correct.