[PATCH] [SVN-4655] configure: add zlib discovery through pkg-config

Hi folks,

attached you'll find a patch for SVN-4655. The configure script was having
problems finding zlib even with a prefix because it is located in /usr/local/lib/hpux32
on HP-UX (--with-zlib=/usr/local won't work). Using pkg-config solves the issue.

Tested with default and not default locations on FreeBSD 9.3 and HP-UX 11.31.

[[[
Fix SVN-4655: configure: add zlib discovery through pkg-config

Rework zlib.m4 in a way that is first searches zlib_prefix, if
given, else search with pkg-config and ultimately in default
locations traversed by the preprocessor and linker.

* build/ac-macros/zlib.m4:
  Add a SVN_ZLIB_PKG_CONFIG function inspired by serf.m4 and
 traverse search by prefix, pkg-config and default locations
]]]

The patch has been produced against trunk.

Subversion client happily works now:
$ file `which svn`
/usr/local/bin/svn: ELF-32 executable object file - IA64

$ svn --version
svn, Version 1.9.4 (r1740329)
   übersetzt am Sep 16 2016, um 22:35:03 auf ia64-hp-hpux11.31

Best regards,

Michael Osipov