Issue #9189: Allow users to set $CFLAGS, $CPPFLAGS, and $LDFLAGS when running

configure to append to Python's default values for those variables, and
similarly allow users to set $XXFLAGS on the make command line to append to the
values set by configure.

In the makefile, this renames the variables that used to be $XXFLAGS to
$PY_XXFLAGS, and renames the old $PY_CFLAGS to $PY_CORE_CFLAGS.  To compensate,
sysconfig now aliases $XXFLAGS=$PY_XXFLAGS so that scripts using it keep
working.  I see that as the right interface, not a backward-compatibility hack,
since these are logically the $XXFLAGS variables; we just use a different name
in the makefile to deal with make's semantics.

Lib/sysconfig.py

@@ -259,6 +259,11 @@ def _parse_makefile(filename, vars=None):
                 # bogus variable reference; just drop it since we can't deal
                 variables.remove(name)
 
+    # Add in CFLAGS, LDFLAGS, and CPPFLAGS, which are named with a
+    # prefix in the Makefile.
+    for var in ('CFLAGS', 'LDFLAGS', 'CPPFLAGS'):
+        done[var] = done['PY_' + var]
+
     # save the results in the global dictionary
     vars.update(done)
     return vars