Now that the build is non-recursive, adding to AM_CPPFLAGS means adding to
_all_ cppflags.
Logical groups of includes have been added instead, and are used individually
by various targets.
Build logic moves from individual Makefile.am's to include files, which
the main src/Makefile.am includes. This avoids having to manage a gigantic
single Makefile.
TODO: Move the rules from the old Makefile.include to where they actually
belong and nuke the old file.
Pieter Wuille