C.2 Release 0.12-2

Bugfix release.

• The mmap configure code was wrong in that, while it wouldn't attempt to call mmap() on systems without a working mmap (`working' according to the AC_FUNC_MMAP test), it would try to compile it, which is incorrect.
• The problem was reported by a user who was building it under cygwin. The ./configure output which he forwarded suggested that the system did have mman.h, but didn't have a `working' mmap -- so it thinks it's got mmap, but autoconf disagrees (I've no idea what autoconf takes as `working'). The actual report was an `invalid cast' error which I've fixed.