Merge r1725325 from trunk:

* Since r1724820 two modules, mod_http2 and mod_ssl, call APACHE_CHECK_OPENSSL,
  with mod_http2 doing it first. Because the result of APACHE_CHECK_OPENSSL
  is cached this causes MOD_LDFLAGS and MOD_CFLAGS to remain unset for mod_ssl
  which in turn causes it not to be linked against Openssl which means that
  mod_ssl cannot be loaded if the Openssl libs haven't been loaded by other means
  already.
  Fix this by caching the values for MOD_LDFLAGS and MOD_CFLAGS created during
  the first run and set them in the cached case.

Submitted by: rpluem
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1729872 13f79535-47bb-0310-9956-ffa450edef68