Add PKG_CHECK_VAR and AS_VAR_COPY compat macros (Courtesy of ycflash)

m4/compat.m4

@@ -0,0 +1,24 @@
+# backwards compat with older pkg-config
+# - pull in AC_DEFUN from pkg.m4
+m4_ifndef([PKG_CHECK_VAR], [
+# 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.
+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
+])
+
+# This hack makes PKG_CHECK_VARS from m4/pkg.m4 work on autoconf 2.63
+# ( courtesy of sunnybear in https://github.com/gdnsd/gdnsd/issues/85 )
+m4_ifndef([AS_VAR_COPY],
+[m4_define([AS_VAR_COPY],
+[AS_LITERAL_IF([$1[]$2], [$1=$$2], [eval $1=\$$2])])])
+## End Autoconf-2.63-Compat