Reduce the overhead of the dispatch table usage.

We now initialize our dispatch table with function pointers that go
and do the rewrite, then we never have to check for a NULL table entry
again.  On my 64-bit build, epoxy_glClear() drops from 83 bytes to 14,
while the total library size only goes up by 5%.

This also paves the way for fixing our dispatch table management using
TLS on windows.
Make func.alias_name always have the name of our alias.

The "None" for "we're not an alias of anything else" wasn't actually
simplifying things.
Move the unwrapped logic to function setup.

I'm going to want the same stuff for ifuncs.
Add a testcase for trying to use libepoxy as a static library.

Note that it will only actually link statically against epoxy if
you've set --enable-static.