Run AC_PROG_GCC_TRADITIONAL only when running AC_PROG_CC,

Run AC_PROG_GCC_TRADITIONAL only when running AC_PROG_CC,
ie, not when user specifies --with-CC.  This corrects a scripting error
that I'm surprised hasn't been reported more often.  Moving the macro call
to the earlier point in the script is correct anyway: if -traditional is needed,
it should get added to CFLAGS before we start using the compiler for
other tests.