Require libxml2 < 2.7

Thread initialization has changed in 2.7, such that VLC and Qt4 cannot
both use libxml2. While this maximum version requirement is bound to
frustrate Linux/BSD users, we cannot fix this within VLC. Falling back
to xtag is not quite as bad as crashing.

configure.ac

@@ -4311,13 +4311,13 @@ dnl
 AC_ARG_ENABLE(libxml2,
   [  --enable-libxml2        libxml2 support (default enabled)])
 AS_IF([test "${enable_libxml2}" != "no"], [
-  PKG_CHECK_MODULES([XML2], [libxml-2.0 >= 2.5], [
+  PKG_CHECK_MODULES([XML2], [libxml-2.0 >= 2.5 libxml-2.0 < 2.7], [
     VLC_ADD_CPPFLAGS([xml],[${XML2_CFLAGS}])
     VLC_ADD_LIBS([xml],[${XML2_LIBS}])
     VLC_ADD_PLUGIN([xml])
   ], [
     AS_IF([test "x${enable_xml2}" != "x"], [
-      AC_MSG_ERROR([Could not find libxml2])
+      AC_MSG_ERROR([Could not find libxml version 2.5 or 2.6])
     ])
   ])
 ])

modules/misc/xml/libxml.c

@@ -35,6 +35,10 @@
 #include <libxml/xmlreader.h>
 #include <libxml/catalog.h>
 
+#if !defined (LIBXML_VERSION) || (LIBXML_VERSION > 20700)
+# error Stale config.cache detected. Erase it and re-run configure.
+#endif
+
 /*****************************************************************************
  * Module descriptor
  *****************************************************************************/