[dunecontrol][CMake][bugfix] Set CMAKE=cmake even without using an option file.

Previously the CMAKE variable was not set if no options file was used. This
fixes this.

bin/dunecontrol

@@ -189,13 +189,14 @@ load_opts() {
         OPTS_FILE_CMAKE="$(eval CMAKE=""; . $DUNE_OPTS_FILE; eval echo \$CMAKE)"
         if test -n "$OPTS_FILE_CMAKE"; then
           CMAKE="$OPTS_FILE_CMAKE"
-	else
-	  # fall back to system cmake
-          CMAKE=cmake
         fi
       fi
     fi
   fi
+  if test -n "$USE_CMAKE" && test -z "$CMAKE"; then
+       # fall back to system cmake
+          CMAKE=cmake
+  fi
   if test "x$DUNE_OPTS_FILE" != "x"; then
     if test "$command" = "configure"; then
       CMAKE_FLAGS="$(. $DUNE_OPTS_FILE; eval echo \$CMAKE_FLAGS)"