Initial version of YaGL Mac support module

Initial version of YaGL Mac support module

This adds a preliminary AGL based implementation of YaGL
platform-specific routines, as well as a few additional changes:
- base EGL types suitables of OS X in eglplatform.h
- changes to both configure and makefiles to enable YaGL and VIGS
- an empty VIGS support module

Both YaGL and VIGS files compile and link, however only YaGL runs
for now. To try it, replace -enable-gl by enable-yagl in the qemu
command line, after installing adequate emulator-yagl and libdrm
packages in the root FS. /etc/virtgl.sh should detect that YaGL
is enabled and switch the EGL libs to the YaGL version