include directories should be added to INCLUDES, not CPPFLAGS

(like it was origintally)...  since AC_TRY_LINK() references
CPPFLAGS but not INCLUDES, we'll add the zlib include directory
to CPPFLAGS *temporarily*


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@92469 13f79535-47bb-0310-9956-ffa450edef68