meson: add and use stamp files for generated headers

meson: add and use stamp files for generated headers

Without using stamp files, meson lists the generated headers as the dependency
for every .c file, bloating build.ninja by more than 2x. Processing all the
dependencies also increases the time to generate build.ninja.

The immediate benefit is that this makes re-configuring and clean builds a bit
faster. The main motivation however is that I have other patches that
introduce additional build targets that further would increase the size of
build.ninja, making re-configuring more noticeably slower.

Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/cgkdgvzdpinkacf4v33mky7tbmk467oda5dd4dlmucjjockxzi@xkqfvjoq4uiy