configure: add macros to stash FLAG variables
configure: add macros to stash FLAG variables

Allow for quick stash/unstash of CPPFLAGS and LDFLAGS.  Library tests
can now be easily bracketted with these macros to allow for values
set in user/site arguments.

Signed-off-by: Ben Walton <6f9de98cc1f3d6d6e7da9953ca2250e07ad38e43@artsci.utoronto.ca>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git