[CMake] Use CHECK_CXX_COMPILER_FLAG_AND_ENABLE_IT() to enable -Werror
[CMake] Use CHECK_CXX_COMPILER_FLAG_AND_ENABLE_IT() to enable -Werror

Mainly for consistency, and because directory property isn't picked up
by `CHECK_CXX_SOURCE_COMPILES()`/etc.

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