Add optional zlib support detection via libxml2 in extconf.

This defines HAVE_ZLIB_H macro when zlib is available through libxml2
dependency.

Author: gemmaro

ext/libxml/extconf.rb

@@ -63,5 +63,10 @@ def crash(str)
     EOL
 end
 
+# Optional zlib support via libxml2; defines HAVE_ZLIB_H if available.
+unless have_header("zlib.h")
+  message "zlib not found: building without compression support\n"
+end
+
 create_header()
 create_makefile('libxml_ruby')