"gallium-egl" disappeared from Poky > 1.7 (because it
disappeared from upstream Mesa, too). Still try to detect
and build it if it exists, but do not fail otherwise.
Change-Id: I01c3988e2da6a4fe66a80e5a5b1262cc6c1da727 Signed-off-by: Manuel Bachmann <manuel.bachmann@iot.bzh>