Fix configure's zlib version check (#2901)

* Fix configure's zlib version check

R wants a version of zlib that is 1.2.5 or newer.

The version checking code just does a lexicographic comparison
of the first 5 characters of the string, so it seesthat the latest
zlib version, 1.2.10, as 1.2.1 and fails.

This patch changes the comparison to use zlibs' hex ZLIB_VERNUM
so that it does not suffer from this problem.

A version of this patch is wending it's way through the R comunity
community and will/should be included in a future release.

I tested the patch with the current R, 3.3.1.

* Tighten zlib dependency version (>= 1.2.5)

* Convert patch to level=1 format.