build: Add -Dui option to Meson
This allows the UI components (libmalcontent-ui and malcontent-control)
to be disabled in the build so that a dependency cycle with flatpak can
be avoided (by building malcontent twice, once with -Dui=disabled
and
then again with -Dui=enabled
).
The dependency graph is: malcontent-control → libmalcontent-ui → flatpak → libmalcontent which becomes cyclic if libmalcontent-ui and libmalcontent can only be built at the same time.
Signed-off-by: Philip Withnall withnall@endlessm.com
Fixes: #16 (closed)