On Windows, it will basically be a copy of the main binaries. On other
OSes, it will be symlinks.
This will help for Windows installers where we want to use major version
only in registry and we need a binary named appropriately for this
apparently.
As for the "major" version, I will in fact use the next major when minor
is 99 (also called API major version), not that it will matter much very
soon!
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.
