build: Use AX_GENERATE_CHANGELOG to generate ChangeLog

This removes some Makefile.am rules and simplifies the build system a
bit.

Signed-off-by: Philip Withnall <philip.withnall@collabora.co.uk>
Reviewed-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D4912