Copy PKG_CHECK_VARS from pkg.m4 from pkg-config 0.28 to...

Copy PKG_CHECK_VARS from pkg.m4 from pkg-config 0.28 to m4/ax_pkg_check_vars.m4 so we have PKG_CHECK_VARS available

m4/ax_pkg_check_vars.m4

+# ax_pkg_check_var.m4 - Macros to locate and utilise variables from pkg-config.            -*- Autoconf -*-
+
+# 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
+])# PKG_CHECK_VAR
+])