configure: reorganize flow of argument checks

configure: reorganize flow of argument checks

Move the argument tests from the 'site overrides' so that they are
ahead of any library tests.  This allows for library tests to take
user specified paths into account.  The intent here is to avoid things
like NO_DEFLATE_BOUND being set due to finding old zlib when the user
has specified an alternate location for zlib.  (Ignore the fact that
properly set *FLAGS can avoid solve this issue.)

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