[3.0] cbad9e1 Use a better way of checking we have pkg-config

[Tollef Fog Heen]

commit cbad9e18b433c9bc009d62ac3e66005076a8dbea
Author: Tollef Fog Heen <tfheen at varnish-software.com>
Date:   Tue Oct 18 20:29:53 2011 +0200

    Use a better way of checking we have pkg-config

diff --git a/configure.ac b/configure.ac
index 1f58e00..1938c1f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -116,7 +116,7 @@ AC_SUBST(NET_LIBS)
 AC_CHECK_LIBM
 AC_SUBST(LIBM)
 
-m4_pattern_forbid([^_?PKG_[A-Z_]+$],[pkg.m4 missing, please install pkg-config])
+m4_ifndef([PKG_PROG_PKG_CONFIG], [m4_fatal([pkg.m4 missing, please install pkg-config])])
 PKG_PROG_PKG_CONFIG
 if test -n $PKG_CONFIG; then
    PKG_CHECK_MODULES([PCRE], [libpcre])