bpo-33483: more correctly handle finding the C compiler (GH-6780)

bpo-33483: more correctly handle finding the C compiler (GH-6780)

Instead of passing configure args such as --without-gcc or --with-icc,
instead prefer to rely on the native way of finding the compiler:
passing CC (or CPP or CXX depending).

This allows configure to find the correct compiler instead of having to
be explicitly told. It also more correctly builds on both macOS and
FreeBSD since the system compiler is used by default (cc)