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.

var/spack/repos/builtin/packages/r/package.py

@@ -63,7 +63,7 @@ class R(Package):
     depends_on('ncurses')
     depends_on('icu4c')
     depends_on('glib')
-    depends_on('zlib@:1.2.8')
+    depends_on('zlib@1.2.5:')
     depends_on('bzip2')
     depends_on('libtiff')
     depends_on('jpeg')
@@ -82,6 +82,8 @@ class R(Package):
     depends_on('pcre')
     depends_on('jdk')
 
+    patch('zlib.patch', when='@:3.3.1')
+
     @property
     def etcdir(self):
         return join_path(prefix, 'rlib', 'R', 'etc')

var/spack/repos/builtin/packages/r/zlib.patch

+*** a/configure	2017-01-21 21:48:35.077000000 +0000
+--- b/configure	2017-01-21 21:50:50.700000000 +0000
+***************
+*** 35496,35505 ****
+  #include <string.h>
+  #include <zlib.h>
+  int main() {
+! #ifdef ZLIB_VERSION
+! /* Work around Debian bug: it uses 1.2.3.4 even though there was no such
+!    version on the master site zlib.net */
+!   exit(strncmp(ZLIB_VERSION, "1.2.5", 5) < 0);
+  #else
+    exit(1);
+  #endif
+--- 35496,35509 ----
+  #include <string.h>
+  #include <zlib.h>
+  int main() {
+! /* Checking ZLIB_VERNUM trick learned here:
+!  * https://github.com/TransitApp/protobuf/blob/master/configure.ac#L95
+!  */
+! #ifdef ZLIB_VERNUM
+!   if (ZLIB_VERNUM < 0x1250) {
+!     exit(1);
+!   }
+!   exit(0);
+  #else
+    exit(1);
+  #endif