Forbid PKG tokens in configure output

Now that we need PCRE to build, we also do need pkg-config at
autoreconf time, so make autoconf complain as early as possible to not
confuse people.

git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@4339 d4fa192b-c00b-0410-8231-f00ffab90ce4

configure.ac

@@ -75,6 +75,7 @@ AC_SUBST(NET_LIBS)
 AC_CHECK_LIBM
 AC_SUBST(LIBM)
 
+m4_pattern_forbid([^_?PKG_[A-Z_]+$],[pkg.m4 missing, please install pkg-config])
 PKG_PROG_PKG_CONFIG
 if test -n $PKG_CONFIG; then
    PKG_CHECK_MODULES([PCRE], [libpcre])