Make the update script more portable

On FreeBSD, "m4" is slightly different to GNU M4, for example,
--prefix-builtins is GNU M4 only feature. GNU M4 is an external package
at /usr/local/bin/gm4. So make it customable variable.
Prevent download db files if not updated yet

This is no effect to production server since the main server update
everyday. However, this is helpful for test sites, which may run update
script several times during developing.