configure: add PKG_CHECK_VAR.m4 to avoid failing on older distros

m4/PKG_CHECK_VAR.m4

@@ -0,0 +1,24 @@
+dnl PKG_CHECK_VAR(VARIABLE, MODULE, CONFIG-VARIABLE,
+dnl [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND])
+dnl -------------------------------------------
+dnl Since: 0.28
+dnl
+dnl Retrieves the value of the pkg-config variable for the given module.
+dnl
+dnl Origin (declared license: GPLv2+ with less restrictive exception):
+dnl https://cgit.freedesktop.org/pkg-config/tree/pkg.m4.in?h=pkg-config-0.29.1#n261
+dnl (AS_VAR_COPY replaced with backward-compatible equivalent and guard
+dnl to prefer system-wide variant by Jan Pokorny <jpokorny@redhat.com>)
+
+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])
+dnl AS_VAR_COPY([$1], [pkg_cv_][$1])
+$1=AS_VAR_GET([pkg_cv_][$1])
+
+AS_VAR_IF([$1], [""], [$5], [$4])dnl
+])dnl PKG_CHECK_VAR
+])dnl m4_ifndef