Only check default installation if dune-common is not found via pkg-config.

Fixes flyspray 1260.
PLEASE REVIEW!


[[Imported from SVN: r7387]]

lib/dunemodules.lib

@@ -167,7 +167,7 @@ setup_control_path() {
   if test -z $DUNE_CONTROL_PATH; then
     DUNE_CONTROL_PATH=.
     # try pkg-config locations
-    if pkg-config dune-common; then
+    if ! pkg-config dune-common; then
       # try usual locations of installed modules
       for i in /usr/local/lib/dunecontrol/ /usr/lib/dunecontrol/; do
         if test -d $i; then