Several of our tools are installed and are used by other projects to
generate code. However, there is no 'install' when projects use glib
as a subproject.

We need some way for glib to 'provide' these tools so that when some
project uses glib as a subproject, find_program('glib-mkenums') will
transparently return the glib-mkenums we just built.

Starting from Meson 0.46, this can be done with the
`meson.override_find_program()` function.

As a bonus, the Meson GNOME module will also use these
'overriden'/'provided' programs instead of looking for them in PATH.