The jpeg_save_markers API has existed since 27th March 1998! So it's
safe to assume that everybody has it, and there's no need to check for
it.

libjpeg-turbo has had a libjpeg.pc since 2016, and the IJG's software
has it too. Dropping support for old libraries is a small price to pay
for the benefits of a simpler build.

!28