[dune-autogen] Prevents overriding am_dir with installed modules.

Before this patch we tried setting am_dir for directory with a
DUNE module. If one is working with local modules
(e.g. dune-common and dune-geometry), that are a subset of installed
modules (e.g. dune-common, dune-geometry, and dune-istl), that are all
installed under the same prefix, then modules that are part of the difference
of the set of installed and the set of the local modules caused am_dir to
point to the am directory of the installed dune-common directory.
After this patch  am_dir is only set once, which fixes this behaviour and
flyspray issue #1420

bin/dune-autogen

@@ -63,20 +63,24 @@ for OPT in "$@"; do
             if test -d "$OPT/m4"; then
               ACLOCAL_FLAGS="$ACLOCAL_FLAGS -I \"$OPT/m4\""
             fi
-            if test -f "$OPT/dune-common.pc.in" ; then
-#            if test \( -d "$OPT/am" \) -a ! \( -h "$OPT/am" \) ; then
-              echo "Found am directory $OPT/am"
-              am_dir="$OPT/am"
-            fi
-            if test -d "$OPT/share/dune/aclocal"; then
-              ACLOCAL_FLAGS="$ACLOCAL_FLAGS -I $OPT/share/dune/aclocal"
-            fi
-            if test -d "$OPT/share/aclocal"; then
-              ACLOCAL_FLAGS="$ACLOCAL_FLAGS -I $OPT/share/aclocal"
-            fi
-            if test -d "$OPT/share/dune-common/am"; then
-              echo "Found am directory $OPT/share/dune-common/am"
-              am_dir="$OPT/share/dune-common/am"
+	    if test -z "$am_dir"; then
+              # if am_dir is already set, then we already find dune-common
+              # and did set am_dir correctly. 
+              if test -f "$OPT/dune-common.pc.in" ; then
+#               if test \( -d "$OPT/am" \) -a ! \( -h "$OPT/am" \) ; then
+                echo "Found am directory $OPT/am"
+                am_dir="$OPT/am"
+              fi
+              if test -d "$OPT/share/dune/aclocal"; then
+                ACLOCAL_FLAGS="$ACLOCAL_FLAGS -I $OPT/share/dune/aclocal"
+              fi
+              if test -d "$OPT/share/aclocal"; then
+                ACLOCAL_FLAGS="$ACLOCAL_FLAGS -I $OPT/share/aclocal"
+              fi
+              if test -d "$OPT/share/dune-common/am"; then
+                echo "Found am directory $OPT/share/dune-common/am"
+                am_dir="$OPT/share/dune-common/am"
+              fi
             fi
 			      PATH=$OPT/bin:$PATH
             ;;