BUILD: makefile: change default value of CC from gcc to cc

BUILD: makefile: change default value of CC from gcc to cc

Change the default value of CC from gcc to cc to be more appropriate
for modern day mix of compilers. On GCC based OS's cc -> gcc. On Clang
based OS's cc -> clang. FreeBSD / OpenBSD have switched to Clang and
this corrects building with the proper compiler on OS's using Clang
as the default compiler. This especially matters for the necessity for
TLS on OpenBSD. I would expect this affects OpenMandriva and other
Linux OS's using Clang as well.