Fix building under cygwin.

Fix building under cygwin.

I attempted to build the latest under cygwin and ran into this:

<snip>
...
creating build/temp.cygwin-1.7.1-i686-2.5
gcc -fno-strict-aliasing -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-
prototypes -I/usr/include/python2.5 -c _hashsplit.c -o b
uild/temp.cygwin-1.7.1-i686-2.5/_hashsplit.o
creating build/lib.cygwin-1.7.1-i686-2.5
gcc -shared -Wl,--enable-auto-image-base build/temp.cygwin-1.7.1-
i686-2.5/_hashsplit.o -L/usr/lib/python2.5/config -lpyt
hon2.5 -o build/lib.cygwin-1.7.1-i686-2.5/_hashsplit.dll
cp build/*/_hashsplit..dll .
cp: cannot stat \ 2uild/*/_hashsplit..dll': No such file or directory
make: *** [_hashsplit.dll] Error 1

</snip>

Some investigation turned up that Makefile was mistakenly referencing
_hashsplit.so instead of _hashsplit.dll

Changing the Makefile to expand the extension macro for the detected
platform, now allows problem free building.