ui/opengl: Reduce build required libraries for opengl

We now use epoxy to load opengl libraries. This means we don't need to
link opengl libraries directly if interfaces handled by epoxy. With
this, we just need epoxy headers and epoxy's *.so to build.

Tested with epoxy-1.3.1.

- sdl2/gtk/console egl stuff doesn't require other than epoxy
- milkymist-tmu2 glx stuff doesn't require other than epoxy

(lm32 test is limited, because can't find mmone-bios.bin, so just test
to load libGL with "./lm32-softmmu/qemu-system-lm32 -M milkymist,accel=qtest")

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>

[ lm32 tested by kraxel ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>