Enabling one backend (Broadway, in commit 1882ff9b) implicitly disabled
all the others, which was probably not intentional. Bring the Autotools
build into line with Meson.
Signed-off-by: Simon McVittie <smcv@debian.org>
getting_started.xml uses relative paths for including code examples
and for some reason the base path is different with meson than with autotools.
Switch both autotools and meson to generate the file and insert the absolute
source path instead.
This also cleans up the content file list: the expand content files have to
be in the content file list as well, so just append them there.
