... to the Doxygen generation log file `documentation/pdfall.log`.
The additions to the log file are intended to find out which parts
take how much time.
Note: use `grep "make_pdf" documentation/pdfall.log` to see the
log output with timestamps.
- documentation/Doxyfile.in: exclude undocumented source files in
src/xutf8/* which reduces parsing by a small amount of time,
estimated about 5 percent of build time. YMMV.
It notably honors SOURCE_DATE_EPOCH if set, making for reproducible
output. For even better reproducibility, use UTC. (Unlike the date
command's output, the result is already always in English.) Extend
this approach to the book, introducing appropriately formatted
PDF_DATE and TODAY variables for its PDF metadata and title page
respectively and making make_pdf configurable.