configure: Warn that cairo-gl and cairo-glesv2 are risky

configure: Warn that cairo-gl and cairo-glesv2 are risky

Most distros do not ship with gl-enabled cairo, since doing so can
result in libgl being linked to each cairo-using client, even if they
don't actually use GL, and this can cause much larger per-client memory
footprint, and thus can become a resource issue.

Furthermore, while in theory this should work fine, we don't actively
test this configuration, and there could be random undiscovered bugs if
it's used.  We keep the option available for people interested in
helping us chase down those issues, but warn everyone else away.