Use different library names on Windows

When we're loading .dll's instead of .so files, use the library filenames
that come in the gtk3 windows binary download.

Also note the definition of g_idle_add - the library was changed from
libgtk to libglib, as it was throwing a 'symbol not found' error.
Remove explicit '.so' for non-windows

This should let things work on OS X as well.
Merge pull request #4 from retupmoca/master

Use different library names on Windows