configure.ac: Use pkg-config to detect libxml2

Use pkg-config to detect libxml2, falling back to xml2-config if the
former is not available.

This patch ensures Asterisk continues to build on systems without
xml2-config installed.

The patch also updates the associated 'configure' files.

ASTERISK-29970 #close

Change-Id: I3c90dfe0b0590486cbb8e6d426a7c5c4199410c0

configure.ac

@@ -737,6 +737,7 @@ fi
 # See if clock_gettime is in librt
 AST_EXT_LIB_CHECK([RT], [rt], [clock_gettime], [])
 
+AST_PKG_CONFIG_CHECK([LIBXML2], [libxml-2.0])
 AST_EXT_TOOL_CHECK([LIBXML2], [xml2-config], , ,
         [#include <libxml/tree.h>
         #include <libxml/parser.h>],

menuselect/configure.ac

@@ -45,6 +45,7 @@ AH_BOTTOM([#endif])
 AC_PROG_CC
 AST_CHECK_GNU_MAKE
 
+PKG_PROG_PKG_CONFIG
 
 AC_LANG(C)
 AC_ARG_ENABLE([debug],
@@ -90,6 +91,7 @@ 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>],