CMake: src-dependencies: do look for openmp 4.0 specifically.
CMake: src-dependencies: do look for openmp 4.0 specifically.

There was a change between openmp3.1 and openmp40
wrt the predetermination of variables.
In particular, that affects const variables.
Since default(none) is used, they "used" to be determined as shared.
Now they won't be.

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