configure.ac: Check for pkg-config presence

If pkg-config is not installed, a very cryptic error occurs when
autogen.sh runs the configure script.  Add a check from:

http://lists.gnu.org/archive/html/automake/2011-03/msg00008.html

to check for pkg-config presence and if pkg-config is not found, a stub
macro disables any configuration of any feature relying on its presence.

README.developer

@@ -1,6 +1,6 @@
 Hamlib - (C) Frank Singleton 2000 (vk3fcs@ix.netcom.com)
          (C) Stephane Fillod 2000-2011
-         (C) The Hamlib Group 2000-2012
+         (C) The Hamlib Group 2000-2013
 
 Take a look at http://sourceforge.net/projects/hamlib/
 Here you will find a mail list, and the latest releases.
@@ -238,6 +238,7 @@ distributions may differ).
 * automake      1.11                        # automake --version
 * libtool       2.2.6b+                     # libtool --version
 * libltdl-dev   2.2.6b+
+* pkg-config    0.25                        # pkg-config --version
 * Git for connection to hamlib.git.sourceforge.net
 
 N.B. Hamlib requires libtool >= 2.2.6b in compliance with CVE-2009-3736.
@@ -260,14 +261,12 @@ Documentation:
 * Doxygen
 
 N.B.:  Some systems can have several versions of the autotools installed. In
-that case, autoconf may be called "autoconf2.50", autoheader
+that case, autoconf may be called "autoconf2.59", autoheader
-"autoheader2.50", and automake "automake-1.7", aclocal "aclocal-1.7" or a
+"autoheader2.59", and automake "automake-1.9", aclocal "aclocal-1.9" or a
 newer version.
 
 IMPORTANT: If autoconf or automake are installed on your system,
 make sure they are matching *at least* the version shown above.
-Some people experience troubles with automake 1.5, if you're one of those,
-it's recommended to upgrade to automake 1.7, which is a lot more stable.
 
 
 1.3. configure and build stage

configure.ac

@@ -321,6 +321,27 @@ AC_ARG_WITH([xml-support],
             [cf_with_xml_support=no])
 AC_MSG_RESULT([$cf_with_xml_support])
 
+
+dnl Check for pkg-config presence and if not installed define a dummy macro
+dnl to disable libxml2 use.  Code borrowed from:
+dnl http://lists.gnu.org/archive/html/automake/2011-03/msg00008.html
+# Check for pkg-config program, used for configuring some libraries.
+#
+m4_define_default([PKG_PROG_PKG_CONFIG],
+    [AC_MSG_CHECKING([pkg-config])
+    AC_MSG_RESULT([no])])
+
+PKG_PROG_PKG_CONFIG
+
+# If the pkg-config autoconf support isn't installed, define its
+# autoconf macro to disable any packages depending on it.
+#
+m4_define_default([PKG_CHECK_MODULES],
+    [AC_MSG_CHECKING([$1])
+    AC_MSG_RESULT([no])
+    $4])
+
+
 AS_IF([test x"${cf_with_xml_support}" = "xyes"], [
       PKG_CHECK_MODULES([LIBXML2],
                         [libxml-2.0],