Append last commit date to version string

Shown in “about” dialog and used in windows packages name.

cmake/getversion.cmake

 find_package(Git)
 if(GIT_FOUND)
 	execute_process(
-		COMMAND ${GIT_EXECUTABLE} describe --long --always
+		COMMAND ${GIT_EXECUTABLE} log -1 --date=short "--pretty=%h (%cd)"
 		WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}
 		RESULT_VARIABLE res_var
 		OUTPUT_VARIABLE GIT_COM_ID