bpo-36235: Fix CFLAGS in distutils customize_compiler() (GH-12236) (GH-12348)

bpo-36235: Fix CFLAGS in distutils customize_compiler() (GH-12236) (GH-12348)

Fix CFLAGS in customize_compiler() of distutils.sysconfig: when the
CFLAGS environment variable is defined, don't override CFLAGS variable with
the OPT variable anymore.

Initial patch written by David Malcolm.

Co-Authored-By: David Malcolm <dmalcolm@redhat.com>
(cherry picked from commit 86082c22d23285995a32aabb491527c9f5629556)