tools: Use libtool --mode=execute

Better fix for !732 (closed).

The original issue was that when create-exports-NetworkManager.sh using nm(1) with the NetworkManager-all-sym wrapper script instead of the actual binary with slibtool. This is because slibtool and GNU libtool do not place the compiled binaries in the same location. The original fix was to test both locations, but this is bit of a hack especially since the build system should not be using the .libs directory directly. However with $(LIBTOOL) --mode=execute this is not a problem since both implementations can figure out where the correct binary is.