Meson is on track to replace autotools as the build system of choice,
so support it in addition to autotools. If all goes well, we'll
eventually be able to drop the latter ...
https://bugzilla.gnome.org/show_bug.cgi?id=783229
When using the meson build system, gvc will be built as a subproject.
As meson restricts submodules to a subprojects/ directory in the
source tree root, move the submodule there.
https://bugzilla.gnome.org/show_bug.cgi?id=783229
