Jon Turney authored Oct 10, 2017 and Adam Jackson committed Dec 13, 2017

Somehow I'd managed to write this with this dependency missing, so this only
works correctly when the generated files already exist and the correct
automatic dependencies generated, but fails on a clean build.

Including generated files with a .c extension into the sources for a target
causes meson to want to compile them (and it seems to be hard to say "make
the directory containing this generated file available to include").

So, change the extension of included generated C fragments to .ic

Update the autotools build to align.

Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>