[release] Make finding *.la files non-fatal.

Actually, having these files lying around does not hurt us.
In addition there are downstream modules that especially create
these for compatibility reasons with autotools. For these
reconfiguring a module using dunecontrol currently aborts with an error

Therefore this commit resorts to just issuing a warning and proceeding
with the compilation instead of aborting with an error.

bin/dunecontrol

@@ -717,8 +717,7 @@ run_default_configure () {
       cd "$ABS_BUILDDIR"
       # check for libtool libs, which might break modules depending on this module
       test -n "`find . -name \*.la`" && \
-          echo "Error: your build directory $ABS_BUILDDIR contains libtool built libraries, please cleanup" && \
-          exit 1
+          echo "Warning: your build directory $ABS_BUILDDIR contains libtool built libraries, please cleanup"
       echo "$PREPARAMS $CMAKE -DCMAKE_MODULE_PATH=\"$CMAKE_MODULE_PATH\" $CMAKE_PARAMS $CMAKE_FLAGS \"$SRCDIR\""
       eval $PREPARAMS $CMAKE "-DCMAKE_MODULE_PATH=\"$CMAKE_MODULE_PATH\" $CMAKE_PARAMS $CMAKE_FLAGS \"$SRCDIR\"" || exit 1
     else