CMake: record proper RAWSPEED_SOURCE_DIR - it's always PROJECT_SOURCE_DIR.
CMake: record proper RAWSPEED_SOURCE_DIR - it's always PROJECT_SOURCE_DIR.

To be noted, there is rawspeed_SOURCE_DIR, but it's lower-case,
which looks uncool.

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