build: Correctly list out generated sources and their dependencies

Sources that are generated during the build should be listed under
BUILT_SOURCES [1]. Their dependencies should be in EXTRA_DIST so that
they can be generated when building from a tarball, and they should
get cleaned by "make clean" like every other artifact produced by
"make".

[1] https://www.gnu.org/software/automake/manual/html_node/Sources.html

https://bugzilla.gnome.org/show_bug.cgi?id=787455