Functional package managers such as GNU Guix rely on environment
variables such as GI_TYPELIB_PATH to discover the system libraries and
resources; extend rather than override them.
* testsuite/introspection/meson.build (env): New variable that extends
rather than override the GI_TYPELIB_PATH and LD_PRELOAD environment
variables.
(api): Use the above as the value of the 'env' keyword argument.
The introspection tests depend on the pygobject module, but we currently
are not checking if it's available at configuration time, which means we
can get build failures like:
> ModuleNotFoundError: No module named 'gi'
when running the test suite.
We hardcoded the typelib directory for only an arch (and a distro),
while we can just get it from gobject-introspection pkg config if tests
are enabled.
