build: use correct CPPFLAGS for out-of-tree build

Generated headers need to be included from the $(builddir).

Also remove unnecessary indirection: AM_CPPFLAGS is used for all
targets that do not have their own foo_CPPFLAGS.

Refs T770, similar to D1571 and D1613.

Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D1646