Makefile: Build on Windows under Cygwin.

Makefile:  Build on Windows under Cygwin.

- Python modules have to end with .dll instead .so to load into Python
  via 'import'.
- GCC under Windows builds all programs with -fPIC, and doesn't accept
  this command-line option.
- libpython2.5.dll is found in /usr/bin under Cygwin (wtf?), so we need
  to add this to the LDFLAGS path.
- 'make clean' should remove .dll files too.