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/Misc/NEWS b/Misc/NEWS
index b63c84c..debfb0c 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -1530,6 +1530,12 @@
 Build
 -----
 
+- Issue #9189: Make a user-specified CFLAGS, CPPFLAGS, or LDFLAGS
+  setting override the configure and makefile defaults, without
+  deleting options the user didn't intend to override.  Developers
+  should no longer need to specify OPT or EXTRA_CFLAGS, although those
+  variables are still present for backward-compatibility.
+
 - Issue #8854: Fix finding Visual Studio 2008 on Windows x64.
 
 - Issue #1759169, #8864: Drop _XOPEN_SOURCE on Solaris, define it for