Build gob2 on Cygwin

Hello,

Here is a patch that allow gob2 to be built on Cygwin (the POSIX
compliant shell for Windows). For Cygwin and MinGW each executable gets
the extension .exe, which is appended by the autotools when they
understand the arguments to bin_PROGRAMS.

I've recently started to use gob2 for Pioneers, a clone of the board
game Settlers of Catan (http://pio.sf.net). It's really easy to create
new classes with gob2.

With kind regards,
Roland Clobus
Developer for Pioneers