Not sure if this is ready for re-test but tried it anyway :-).
Had to set `-Dgtkdoc=false`, what tool is needed for that?
Geany plugins now build if configured `--prefix=/home/lex/geany_meson --with-geany-libdir=/home/lex/geany_meson/lib` and install to `$PREFIX/lib/geany` but do not appear in plugin manager.
The plugins that are part of geany are installed in `$PREFIX/lib/x86_64-linux-gnu/geany` and do appear, but geany-plugins won't configure with `--with-geany-libdir=/home/lex/geany_meson/lex/x86_64-linux-gnu`.
So it looks like another mixed up libdir problem.