environment().prepend() does not stack. None of the operations on that
object stack. This is documented in the Meson reference manual. Also
avoid overriding previously-set operations because that will cause
a warning in a future version of Meson.
Also, we do not need to manually construct MONO_PATH values. We can
pass an array and have Meson construct it for us.
When gstreamer-sharp is used as a subproject (e.g. gst-build) and the
master project is built with default-library=both, gstreamer-sharp
should still only build shared libraries instead of failing to
configure.
https://bugzilla.gnome.org/show_bug.cgi?id=794656
