Fix warning message when libX11.so.* are not found.

We complain that we can't find "X development files", but the reverse
is true - if we had X development files, we'd find libX11.so (which
we aren't looking for). What we look for (and find) are the runtime files.
Don't do the soname checking dance for Xinerama.

Xinerama has only ever had a soname of 1.

Checking for 1, 2, and 3 is just weird and pointless. And by not using a
default value, we duplicate the bug which add24aaa1 fixed.
Merge pull request #1183 from directhex/xinerama

Don't do the soname checking dance for Xinerama.