https://archlinux.org/todo/broken-nvchecker-configurations/

Signed-off-by: Morten Linderud <foxboron@archlinux.org>

Add upstream's pre-amalgamated sources, as the release tarballs are now not shipping the amalgamate binary anymore.
Split out the docs to qcustomplot-docs.
Simplify installation using find.

Removing custom postprocessing for doxygen documentation (FS#63554). Adding libqcustomplot.so to provides.

Adding qcustomplot (needed for the next polyphone release), in replacement of qcustomplot-qt5 (which has wrong naming).