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.
diff --git a/Modules/makesetup b/Modules/makesetup
index 23f778d..40dfa9d 100755
--- a/Modules/makesetup
+++ b/Modules/makesetup
@@ -219,7 +219,7 @@
 			case $doconfig in
 			no)	cc="$cc \$(CCSHARED) \$(CFLAGS) \$(CPPFLAGS)";;
 			*)
-				cc="$cc \$(PY_CFLAGS)";;
+				cc="$cc \$(PY_CORE_CFLAGS)";;
 			esac
 			rule="$obj: $src; $cc $cpps -c $src -o $obj"
 			echo "$rule" >>$rulesf