[AD] statically linking small addons

[Peter Wang <novalazy@xxxxxxxxxx>]

Matthew suggested that small addons should be statically linked on
Windows so as to avoid proliferation of DLLs. (I think that's not
necessarily on Linux; don't know about Mac.)

Indeed it's not very hard to accomodate that in CMake.  However, our
static libraries have the "-static" suffix so users on Windows will need
to write -lallegro-4.9 but -lallegro_color-static-4.9 (or whatever).
Is that okay?

Secondly, ALLEGRO_STATICLINK.  I'm fuzzy on what it does exactly so
would someone explain if there will be any trouble with that?

Peter