When we build GIMP with `-Dauto_features=enabled` to ensure we have
fully-featured GIMP and to have it warn us when we are missing some
optional dependency, Meson also enables relocatable bundle.
This is unexpected since being relocatable is a binary property rather
than a feature. This patch switches it option to a combo type to make
it ignore auto_features option.
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.
Jehan