libxml2 knows too much about zlib internals. Beginning from zlib 1.2.2.3

the zlib authors has provided a 'gzdirect' function to detect whether the
current gzFile descriptor is still providing a stream from uncompression,
so use it instead of rolling own.

The upcoming zlib 1.2.4 update will break the current libxml2 usage.

Patch was the same as Mark Adler provided to libxml2 maintainers at:

	http://osdir.com/ml/svn-commits-list/2010-01/msg05723.html

PR:		ports/144828
Approved by:	mezz (freebsd-gnome@)

textproc/libxml2/Makefile

@@ -12,7 +12,7 @@
 
 PORTNAME=	libxml2
 PORTVERSION=	2.7.6
-PORTREVISION?=	1
+PORTREVISION?=	2
 CATEGORIES?=	textproc gnome
 MASTER_SITES=	ftp://fr.rpmfind.net/pub/libxml/ \
 		ftp://gd.tuwien.ac.at/pub/libxml/ \

textproc/libxml2/files/patch-xmlIO.c

@@ -0,0 +1,20 @@
+--- ./xmlIO.c.orig	2009-09-24 08:32:00.000000000 -0700
++++ ./xmlIO.c	2010-03-17 12:35:00.957293884 -0700
+@@ -2518,6 +2518,9 @@
+ #ifdef HAVE_ZLIB_H
+ 	if ((xmlInputCallbackTable[i].opencallback == xmlGzfileOpen) &&
+ 		(strcmp(URI, "-") != 0)) {
++#if defined(ZLIB_VERNUM) && ZLIB_VERNUM >= 0x1230
++	    ret->compressed = !gzdirect(context);
++#else
+ 	    if (((z_stream *)context)->avail_in > 4) {
+ 	        char *cptr, buff4[4];
+ 		cptr = (char *) ((z_stream *)context)->next_in;
+@@ -2529,6 +2532,7 @@
+ 		    gzrewind(context);
+ 		}
+ 	    }
++#endif
+ 	}
+ #endif
+     }