build/autotools: generate "config-extra.h" via makefile "config-extra.h.mk"

When the code that generates "config-extra.h" changes, we want to regenerate
the file. Move that code to a separate makefile so we can add a
dependency.

Otherwise, we'd had to depend on "Makefile", which itself is generated by
Makefile.am.

Also, depend on "config.h" to regenerate it when ./configure runs and
touches that header. This may not cover all cases where ./configure's
configuration changes and a regeneration would be due. But such is life.

Also, most components depend on this header, so let various .dirstamp
files depend on it, so we are sure to build this first. That because,
autotools generates dependencies for header files automatically, but
that requires that the header file exist. Such automatic dependencies
don't work out-of-the-box for generated headers.

Co-authored-by: Thomas Haller <thaller@redhat.com>

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