False positive caused by PNGs

[benegee]

In our repository https://github.com/trixi-framework/libtrixi we have doxygen's Doxyfile lying around. Since 1.20 we get the following error:

error: `PN` should be `ON`
  --> ./docs/doxygen/Doxyfile:1767:71
     |
1767 | # https://inkscape.org)/ to generate formulas as SVG images instead of PNGs for
     |                                                                        ^^

[epage]

Closing as a duplicate of #967