CMake: Fix scmversion.cpp always updating on Mac

src/scmversion/CMakeLists.txt

@@ -1,15 +1,19 @@
-# _scmversion.cpp is a dummy file that will never be created, so the command will always be run
 if(CMAKE_HOST_SYSTEM MATCHES "Windows")
-  add_custom_command(OUTPUT scmversion.cpp _scmversion.cpp
-                     COMMAND cmd /k "${CMAKE_CURRENT_SOURCE_DIR}/gen_scmversion.bat"
-                     WORKING_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}")
+  add_custom_target(generate_scmversion
+                    COMMAND cmd /k "${CMAKE_CURRENT_SOURCE_DIR}/gen_scmversion.bat"
+                    WORKING_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}"
+                    BYPRODUCTS "${CMAKE_CURRENT_BINARY_DIR}/scmversion.cpp"
+                    COMMENT "Checking if scmversion.cpp needs to be updated")
 else()
-  add_custom_command(OUTPUT scmversion.cpp _scmversion.cpp
-                     COMMAND "${CMAKE_CURRENT_SOURCE_DIR}/gen_scmversion.sh")
+  add_custom_target(generate_scmversion
+                    COMMAND "${CMAKE_CURRENT_SOURCE_DIR}/gen_scmversion.sh"
+                    WORKING_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}"
+                    BYPRODUCTS "${CMAKE_CURRENT_BINARY_DIR}/scmversion.cpp"
+                    COMMENT "Checking if scmversion.cpp needs to be updated")
 endif()
 
 add_library(scmversion
-  scmversion.cpp
+  "${CMAKE_CURRENT_BINARY_DIR}/scmversion.cpp"
   scmversion.h
 )
-
+add_dependencies(scmversion generate_scmversion)