"mandatory" means that it is required even if you disable all optional
features using --without-<package>.
while for libpng, tcltk I can see that a default of "yes" is desirable,
should this be true for lesser used features?
e.g.
--with-postgres support PostgreSQL functionality (default: yes)
--with-fftw support FFTW functionality (default: yes)
(mostly speaking of postgres here. why is it treated differently than
MySQL?)
Enabling optional features by default has historically been a
defensive mechanism against users who basically run
"./configure ; make ; make install" then complain that the modules
which they didn't enable don't seem to work.
If we enable most things by default, and force them to disable things
manually, they're more likely to realise that the problem is due to
the feature being disabled.