CMake: Always run gen_scmversion script on rebuilds

Fixes issue where scmversion.cpp would not be regenerated with updated
Git info on rebuilds, causing an outdated version to be displayed in
window titles.

src/scmversion/CMakeLists.txt

@@ -1,9 +1,10 @@
+# _scmversion.cpp is a dummy file that will never be created, so the command will always be run
 if(WIN32)
-  add_custom_command(OUTPUT scmversion.cpp
+  add_custom_command(OUTPUT scmversion.cpp _scmversion.cpp
                      COMMAND cmd /k "${CMAKE_CURRENT_SOURCE_DIR}/gen_scmversion.bat"
                      WORKING_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}")
 else()
-  add_custom_command(OUTPUT scmversion.cpp
+  add_custom_command(OUTPUT scmversion.cpp _scmversion.cpp
                      COMMAND "${CMAKE_CURRENT_SOURCE_DIR}/gen_scmversion.sh")
 endif()