Fix for bug #1525447 (renaming to MacOSmodule.c would also work, but not

without causing problems for anyone that is on a case-insensitive filesystem).

Setup.py tries to compile the MacOS extension from MacOSmodule.c, while the
actual file is named macosmodule.c. This is no problem on the (default)
case-insensitive filesystem, but doesn't work on case-sensitive filesystems.