build: Don’t install things if used as a subproject
When used as a subproject, we expect to be built as a static library,
and not distributed in any recognisable form as libglib-testing. So
don’t install libglib-testing things when
ninja install is run for the
module which includes libglib-testing as a subproject.
Signed-off-by: Philip Withnall firstname.lastname@example.org