Rearrange c.h to create a "compiler characteristics" section.

Generalize section 1 to handle stuff that is principally about the
compiler (not libraries), such as attributes, and collect stuff there
that had been dropped into various other parts of c.h.  Also, push
all the gettext macros into section 8, so that section 0 is really
just inclusions rather than inclusions and random other stuff.

The primary goal here is to get pg_attribute_aligned() defined before
section 3, so that we can use it with int128.  But this seems like good
cleanup anyway.

This patch just moves macro definitions around, and shouldn't result
in any changes in generated code.

Back-patch of commit 91aec93e.

Discussion: https://postgr.es/m/20171110185747.31519.28038@wrigleys.postgresql.org