build: switch from $(INCLUDES) to $(AM_CPPFLAGS) to make automake happy

build: switch from $(INCLUDES) to $(AM_CPPFLAGS) to make automake happy

Unfortunately, $(AM_CPPFLAGS) gets overridden by per-target _CPPFLAGS
variables, which $(INCLUDES) did not, so this requires some additional
changes.

In most places, I have just gotten rid of the per-target _CPPFLAGS
variables; in directories with a single target, the per-target
variable is unnecessary, and in directories with multiple targets, the
per-target variable is often undesirable, since it forces some files
to be compiled twice, even though there ends up being no difference
between the two files.