Fix llvm search for PCBSD

Apparently PCBSD uses llvm-config34, without the dash before "34"
Make pocl more friendly with other ICD loaders than ocl-icd

- When ocl-icd is detected, tests will use it and no libOpenCL is built.
- When other ICD loader is detected, libOpenCL is built but not installed,
  and tests will use direct linkage to the built libOpenCL.
- No ICD loader means libOpenCL is built and also installed.
Fix the uppercase of some commands to match the rest