738dab0fce
It must not be a boolean but a `feature` option, with `auto` by default. `auto` value mean enabled for macOS and Win32, and disabled for other cases. This default logics disappeared in the meson build. Also the mypaint-brushes package is a mandatory dependency, which must always be checked. Absence is fatale. Finally properly set the MYPAINT_BRUSHES_DIR macro depending on the proper relocatable case.
