autogen: use AM_CPPFLAGS iso INCLUDES
autogen: use AM_CPPFLAGS iso INCLUDES

automake-1.12.4 warns about using the deprecated INCLUDES, and having to
use AM_CPPFLAGS instead

Since INCLUDES is now mapped onto something for both MSVC, as well as
automake, we just stick to INCLUDES in our Makefile.ag files and
translate it into AM_CPPFLAGS for automake.  Since we don't do
per-target includes currently, we can't use the *_CPPFLAGS that automake
also supports.

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