[4.1] 83c1c54 Display the correct error if varnish.pc is missing

[Lasse Karstensen]

commit 83c1c54e9385df740d9a14158c055e4a343acca9
Author: Federico G. Schwindt <fgsch at lodoss.net>
Date:   Thu Jan 21 12:39:36 2016 +0000

    Display the correct error if varnish.pc is missing
    
    Discussed with mithrandir at .

diff --git a/varnish.m4 b/varnish.m4
index 8a9f647..105a224 100644
--- a/varnish.m4
+++ b/varnish.m4
@@ -42,7 +42,7 @@ m4_pattern_forbid([^_?VARNISH[A-Z_]+$])
 m4_pattern_allow([^VARNISH_VMOD(_INCLUDE_DIR|TOOL)$])
 # Check for pkg-config
 PKG_CHECK_EXISTS([varnishapi],[],[
-	if test -n "$PKG_CONFIG"; then
+	if test -z "$PKG_CONFIG"; then
 		AC_MSG_FAILURE(
 [The pkg-config script could not be found or is too old.  Make sure it
 is in your PATH or set the PKG_CONFIG environment variable to the full