Doxygen doesn't support incremental builds. It could be ok because it
doesn't take much time in this codebase. However it's not because it
makes old output look new which has a cascade effect on sphinx:
https://sourceforge.net/p/doxygen/mailman/message/36580807/
Make doxygen artificially smarter by saving and restoring modify
timestamps on the filesystem for doxygen output files that haven't
changed.
On my system this brings down the time to run an incremental "make
htmldocs" from 75s down to 8s (cmake -DKCONFIG_TURBO_MODE=1 used in both
cases)
This optimization speeds-up non-doxygen documentation work only.
Signed-off-by: Marc Herbert <marc.herbert@intel.com>
Marc Herbert