utils: Enable __atomic* built-ins based on the GCC version

utils: Enable __atomic* built-ins based on the GCC version

This solves a problem with GNAT when compiling charon-tkm as __atomic*
built-ins are only provided in GCC 4.7 and newer.

Currently GNAT 4.6 and GCC 4.7.2 is shipped with Debian wheezy (stable),
as used in the testing environment.  So while the configure script correctly
detected the __atomic* built-ins, and defined HAVE_GCC_ATOMIC_OPERATIONS,
this define turned out to be incorrect when charon-tkm was later built
with GNAT.