Since commit dc701d6 in the OpenWrt packages repo the host libxml2 package
provides a static lib only. But Asterisk does not check that and calls
pkg-config without "--static". The result is that menuselect doesn't build.

So don't use pkg-config for the libxml2 detection. Asterisk will resort to
use xml2-config, which outputs all the flags needed.

--- a/menuselect/configure.ac
+++ b/menuselect/configure.ac
@@ -91,7 +91,6 @@ else
   AST_EXT_LIB_CHECK([TINFO], [tinfo], [keypad], [curses.h])
 fi
 
-AST_PKG_CONFIG_CHECK([LIBXML2], [libxml-2.0])
 AST_EXT_TOOL_CHECK([LIBXML2], [xml2-config], , ,
         [#include <libxml/tree.h>
         #include <libxml/parser.h>],