configure.ac:1349: error: possibly undefined macro: PKG_CONFIG_LIBDIR

Obviously this is not a problem with pkg-config 0.26 but older versions
seem to show this.

Fix suggested by: Kamil Dudka
Reported by: Guenter
Bug: http://curl.haxx.se/mail/lib-2011-11/0298.html

---
  configure.ac |    4 ++++
  1 files changed, 4 insertions(+), 0 deletions(-)
diff --git a/configure.ac b/configure.ac
index 52158f1..7d2c7e4 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2377,6 +2377,10 @@ case "$LIBIDN" in
         fi
         if test "x$idn" != "xyes"; then
+
+         dnl to prevent errors with pkg-config < 0.26
+         m4_pattern_allow(PKG_CONFIG_LIBDIR)
+
           dnl check with pkg-config
           PKG_CHECK_MODULES(LIBIDN_PC, libidn >= 0.0.0, [idn=yes], [idn=no])
           if test "x$idn" = "xyes"; then
-- 
1.7.7.3
-- 
  / daniel.haxx.se
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