The git describe command in CMakeLists.txt doesn't work when compiling out-of-source

[drebatto]

The git describe command in CMakeLists.txt:

execute_process(COMMAND git describe
                WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
                OUTPUT_VARIABLE GIT_TAG
                ERROR_QUIET)

works only when issued from the .git directory.
The WORKING_DIRECTORY should point to the .git directory instead of the current directory.