build: Simplify libjpeg detection
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.
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