[configure] Allow compiling against lablgtk2 even if lablgtk3 is also installed

configure.in

@@ -921,26 +921,41 @@ new_section "configure tools and libraries used by some plug-ins"
 # lablgtk2
 ##########
 
+define([ENABLE_LABLGTK3_HELP],
+  AC_HELP_STRING([--disable-lablgtk3],
+  [in case lablgtk2 and lablgtk3 are available, the default is to compile
+   against lablgtk3. Use this option to force compiling against lablgtk2]))
+
+AC_ARG_ENABLE(
+  lablgtk3,[ENABLE_LABLGTK3_HELP],
+  [ENABLE_LABLGTK3=$enableval],[ENABLE_LABLGTK3=yes])
+
 REQUIRE_LABLGTK="$REQUIRE_LABLGTK$REQUIRE_GNOMECANVAS"
 USE_LABLGTK="$USE_LABLGTK$USE_GNOMECANVAS"
 
-  LABLGTK_PATH=`ocamlfind query lablgtk3 | tr -d '\\r\\n'`
-  if test "$LABLGTK_PATH" = ""; then
-    LABLGTK_VERSION=2
-    LABLGTK_PATH=`ocamlfind query lablgtk2 | tr -d '\\r\\n'`
-    if test "$LABLGTK_PATH" = "" -o "$LABLGTK_PATH" -ef "$OCAMLLIB/lablgtk2" ; then
+LABLGTK_PATH=""
+
+if test "$ENABLE_LABLGTK3" = "yes"; then
+  LABLGTK_PATH=`ocamlfind query lablgtk3 | tr -d '\\r\\n'`;
+fi
+
+if test "$LABLGTK_PATH" = ""; then
+  LABLGTK_VERSION=2
+  LABLGTK_PATH=`ocamlfind query lablgtk2 | tr -d '\\r\\n'`
+  if test "$LABLGTK_PATH" = "" -o \
+          "$LABLGTK_PATH" -ef "$OCAMLLIB/lablgtk2" ; then
        echo "Ocamlfind -> using +lablgtk2.($LABLGTK_PATH,$OCAMLLIB/lablgtk2)"
        LABLGTK_PATH=+lablgtk2
        LABLGTKPATH_FOR_CONFIGURE=$OCAMLLIB/lablgtk2
-    else
+  else
        echo "Ocamlfind -> using $LABLGTK_PATH"
        LABLGTKPATH_FOR_CONFIGURE=$LABLGTK_PATH
-    fi;
-  else
-    LABLGTK_VERSION=3
-    echo "ocamlfind -> using $LABLGTK_PATH"
-    LABLGTKPATH_FOR_CONFIGURE=$LABLGTK_PATH;
-  fi
+  fi;
+else
+  LABLGTK_VERSION=3
+  echo "ocamlfind -> using $LABLGTK_PATH"
+  LABLGTKPATH_FOR_CONFIGURE=$LABLGTK_PATH;
+fi
 
 configure_library([GTKSOURCEVIEW],
                   [$LABLGTKPATH_FOR_CONFIGURE/lablgtksourceview2.$LIB_SUFFIX,