Modify zlib check to fail if zlib.h is not found either
Modify zlib check to fail if zlib.h is not found either

* configure.in: Fail if zlib header check fails.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion