Improve function Fl_X11_System_Driver::newUUID()

[Manolo]

I'd like to improve function Fl_X11_System_Driver::newUUID()

that generates UUIDs having it call function uuid_generate() from libuuid

when it's found at run-time. The code falls back to the present ad hoc method

when uuid_generate() isn't available.

That's in attached uuid_generate.patch.txt

The modified code can be tested adding

     fprintf(stderr, "UUID=%s\n", Fl_Preferences::newUUID());

as first statement in main() of test/preferences.cxx