Improve wording for the OpenCL blacklist/whitelist

Change-Id: I4fd0a255903daab3d5a1888eeff266f831ad590b
No need to have OpenCL optional at configure-time

It continues to not be used on platforms where not available (iOS and
Android), and on others it is optional at run-time anyway.

Change-Id: I7ba2f87ac143aad78df7edc2e6e6dc014c29c071
No need to have OpenGL slide transitions optional at configure-time

(The --disable-opengl option was misnamed; it actually referred
only to the OpenGL slide transitions, not our other use of OpenGL.)

Use of OpenGL is still hardcoded off for platforms where we don't know whether
and/or how to handle it yet (iOS and Android), and otherwise it is optional at
run-time.

Change-Id: I65d0612dbc79ee07273854e5a468d891ef07c6b7