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.
