diff --git a/pkg.m4 b/pkg.m4
index a6451d8..a63b535 100644
--- a/pkg.m4
+++ b/pkg.m4
@@ -26,7 +26,7 @@
 # ----------------------------------
 AC_DEFUN([PKG_PROG_PKG_CONFIG],
 [m4_pattern_forbid([^_?PKG_[A-Z_]+$])
-m4_pattern_allow([^PKG_CONFIG(_PATH)?$])
+m4_pattern_allow([^PKG_CONFIG(_PATH|_LIBDIR)?$])
 AC_ARG_VAR([PKG_CONFIG], [path to pkg-config utility])
 AC_ARG_VAR([PKG_CONFIG_PATH], [directories to add to pkg-config's search path])
 AC_ARG_VAR([PKG_CONFIG_LIBDIR], [path overriding pkg-config's built-in search path])