pythongh-96398: Improve accuracy of compiler checks in configure.ac (python#117815)

The following variables are now used in compiler checks:
- $ac_cv_gcc_compat is set to 'yes' for GCC compatible compilers
  (the C preprocessor defines the __GNUC__ macro)
- for compiler basename checks, use $CC_BASENAME
  (may contain platform triplets)
- for the rest, use $ac_cv_cc_name
  (does not contain platform triplets)

Author: erlend-aasland