[CMake] compiler-versions.cmake: bump required clang version up to 3.9
[CMake] compiler-versions.cmake: bump required clang version up to 3.9

This way, both the gcc (5), and clang (3.9) have full openmp 4.0 support.

LebedevRI/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed,darktable-org/rawspeed,darktable-org/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed