Add support for __COUNTER__.

__COUNTER__ allows one to obtain incrementing (read: unique) numbers
from the C preprocesor. This is useful when implementing things like a
robust implementation of CTASSERT(), which currently fails when using
it more than once on a single line of code. Probably not likely to cause
any breakage, but still.

__COUNTER__ was also added to GCC 4.3, but since that implementation is
GPLv3 licensed, I took the liberty of implementing it without looking at
any upstream sources. Therefore, this version is licensed under the same
license as the rest of the code; GPLv2.