[AD] 4.4 C-only by default

[Peter Wang <novalazy@xxxxxxxxxx>]

Given the progress we've made on Windows, I think we can make the C-only
port the default on x86 platforms as well from 4.3 onwards.  I don't
know that we should actually delete it, but if the asm port if selected
the library should be given a different name.

We should also remove the SIGALRM support from the Unix port, as it
would interfere with AllegroGL.

Peter