Background OpenCL compilation re-factoring

Don't use a mutex and condition in each ScFormulaCellGroup.

Not sure at all if this crack works, and I hate every moment spent on it, but
at least it is still behind an environment variable.

Change-Id: I70c917d47112e3a2ed010de4d123a4155f71c004
Avoid assertion failure if group has been disabled

Change-Id: I918f2716778ddd9ede56b5df731a531f97a3b24d
Add some more logging

Change-Id: I637c365626242142a0f6fc0ac002fa9dd5f1340a
Drop the check of the SC_BACKGROUND_COMPILATION env var

Change-Id: Ia33002819023c79160d921fbc1cca7a5c12d143a