Define PKG_CHECK_VAR for legacy systems

varnish-legacy.m4

 # varnish-legacy.m4 - Macros to locate Varnish header files. -*- Autoconf -*-
-# serial 3 (varnish-4.0)
+# serial 4 (varnish-4.0)
 
-# Copyright (c) 2013-2015 Varnish Software AS
+# Copyright (c) 2013-2016 Varnish Software AS
 # All rights reserved.
 #
 # Author: Tollef Fog Heen <tfheen@varnish-software.com>
@@ -33,6 +33,23 @@ m4_ifndef([AS_VAR_COPY],
   [m4_define([AS_VAR_COPY],
      [AS_LITERAL_IF([$1[]$2], [$1=$$2], [eval $1=\$$2])])])
 
+# backward compatibility with older pkg-config
+# PKG_CHECK_VAR(VARIABLE, MODULE, CONFIG-VARIABLE,
+# [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND])
+# -------------------------------------------
+# Retrieves the value of the pkg-config variable for the given module.
+m4_ifndef([PKG_CHECK_VAR], [
+AC_DEFUN([PKG_CHECK_VAR],
+[AC_REQUIRE([PKG_PROG_PKG_CONFIG])dnl
+AC_ARG_VAR([$1], [value of $3 for $2, overriding pkg-config])dnl
+
+_PKG_CONFIG([$1], [variable="][$3]["], [$2])
+AS_VAR_COPY([$1], [pkg_cv_][$1])
+
+AS_VAR_IF([$1], [""], [$5], [$4])dnl
+])
+])
+
 # VARNISH_VMOD_INCLUDE_DIR([])
 # ----------------------------