CMake: stop overwriting branding settings in the build dir

- Only copy over branding files if they are newer

Typically I have KDevelop open while working on Calamares; if I
am editing settings in `branding.desc` in the build directory,
then every time KDevelop runs CMake in the background, my
changes (for testing branding things!) would be overwritten.
Don't do that.

For normal builds with a clean build directory, this doesn't change
anything since the target is missing; changing a file in the
source directory **will** copy it over the build directory version.

CMakeModules/CalamaresAddBrandingSubdirectory.cmake

@@ -70,7 +70,11 @@ function( calamares_add_branding NAME )
         foreach( BRANDING_COMPONENT_FILE ${BRANDING_COMPONENT_FILES} )
             set( _subpath ${_brand_dir}/${BRANDING_COMPONENT_FILE} )
             if( NOT IS_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/${_subpath} )
-                configure_file( ${_subpath} ${_subpath} COPYONLY )
+                set( _src ${CMAKE_CURRENT_SOURCE_DIR}/${_subpath} )
+                set( _dst ${CMAKE_CURRENT_BINARY_DIR}/${_subpath} )
+                if( ${_src} IS_NEWER_THAN ${_dst} )
+                    configure_file( ${_src} ${_dst} COPYONLY )
+                endif()
 
                 install( FILES ${CMAKE_CURRENT_BINARY_DIR}/${_subpath}
                             DESTINATION ${BRANDING_COMPONENT_DESTINATION}/${_subdir}/ )