Remove now-unneeded test for GCC CAS from extconf.rb

Previously, there was a preprocessor conditional directive in
atomic_reference.c which checked for HAVE_GCC_CAS. It's gone now.
So there is no reason to waste time while compiling.
#get/#set have 'volatile' semantics when compiled with GCC
#get/#set have 'volatile' semantics when compiled with MS Visual C++
#get/#set have 'volatile' semantics when compiled on Mac OS X
#swap also has 'volatile' semantics
Merge pull request #4 from alexdowad/volatile_semantics_for_get_set

Volatile semantics for #get, #set, and #swap