Define FFI functions in libgdkglext, provide dll names for windows

This commit declares gdkglext ffi functions as part of the libgdkglext library, which is separate from the libgtkglext one. This fixes an "undefined alien" error on SBCL on windows, and looks like the right thing to do for other platforms as well (gtkglext builds two separate libs on Linux, but I don't have a Mac environment available to test).

This also provides the proper gtkglext dll names on windows.