Makefile: dependency on BYPRODUCTS not hooked up
add_custom_command(OUTPUT file1 file2 ...) to generate two files. The command may not update
file2 if it shouldn't be updated. However
file2 is always touched and this leads to rebuild of dependent targets. This behavior is introduced in 66a9c90c.
file2 should not be explicitly touched. Its command responsibility to update it if necessary.