configure: Better handling of missing pkgconfig

configure.ac

@@ -15,6 +15,7 @@ AM_SILENT_RULES([yes])
 
 AX_IS_RELEASE([minor-version])
 AX_CHECK_ENABLE_DEBUG([yes])
+AX_REQUIRE_DEFINED([PKG_PROG_PKG_CONFIG])
 
 AC_USE_SYSTEM_EXTENSIONS
 AM_MAINTAINER_MODE([enable])
@@ -27,6 +28,7 @@ AM_PROG_AR
 LT_PREREQ([2.2.6])
 LT_INIT([disable-static])
 AC_PATH_PROG(MDTOOL, mdtool, no)
+PKG_PROG_PKG_CONFIG
 
 dnl -----------------------------------------------------------
 dnl Language Support

m4/ax_require_defined.m4

@@ -0,0 +1,37 @@
+# ===========================================================================
+#    http://www.gnu.org/software/autoconf-archive/ax_require_defined.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_REQUIRE_DEFINED(MACRO)
+#
+# DESCRIPTION
+#
+#   AX_REQUIRE_DEFINED is a simple helper for making sure other macros have
+#   been defined and thus are available for use.  This avoids random issues
+#   where a macro isn't expanded.  Instead the configure script emits a
+#   non-fatal:
+#
+#     ./configure: line 1673: AX_CFLAGS_WARN_ALL: command not found
+#
+#   It's like AC_REQUIRE except it doesn't expand the required macro.
+#
+#   Here's an example:
+#
+#     AX_REQUIRE_DEFINED([AX_CHECK_LINK_FLAG])
+#
+# LICENSE
+#
+#   Copyright (c) 2014 Mike Frysinger <vapier@gentoo.org>
+#
+#   Copying and distribution of this file, with or without modification, are
+#   permitted in any medium without royalty provided the copyright notice
+#   and this notice are preserved. This file is offered as-is, without any
+#   warranty.
+
+#serial 1
+
+AC_DEFUN([AX_REQUIRE_DEFINED], [dnl
+  m4_ifndef([$1], [m4_fatal([macro ]$1[ is not defined; is a m4 file missing?])])
+])dnl AX_REQUIRE_DEFINED