CMake: hopefully properly disable -Wunreachable-code diagnostic
CMake: hopefully properly disable -Wunreachable-code diagnostic

Compiler-version-based check is too fraglie.

Fixes https://github.com/darktable-org/rawspeed/issues/104

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