configure: wrap some library tests with GIT_STASH_FLAGS

configure: wrap some library tests with GIT_STASH_FLAGS

Libraries that can have user specificed base paths are wrapped with
GIT_STASH_FLAGS/GIT_UNSTASH_FLAGS to ensure that the proper versions
on the system are tested.  This ensures, for example, that the zlib
tests for deflateUnbound are done with the version of zlib requested
by the user.  This is most useful in the absence of good settings for
CPPFLAGS and/or LDFLAGS.

Signed-off-by: Ben Walton <bwalton@artsci.utoronto.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>