libgbm: remove backends we don't use

Mesa sets gbm_dri as builtin. We don't have it here which causes:

undefined symbol: gbm_dri_backend

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
backend_pvr_omap3: add debug to see if backend was found

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
wsegl: use autotools results for wayland libs

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
do not pass -no-undefined to plugins - that helps us to find undefined symbols during build

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
libgbm: add more debug

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>