In Automake, CPPFLAGS stands for C PreProcessor flags. C source files are
compiled with CPPFLAGS and CFLAGS, while C++ source files are compiled
with CPPFLAGS and CXXFLAGS. The exception to this naming convention is
the PKG_CHECK_MODULES macro which substs a variable named <FOO>_CFLAGS
which should properly be called <FOO>_CPPFLAGS.

This untangles all the flags definitions so that -I and -D flags, which
are preprocessor arguments, are put into CPPFLAGS variables, not into
CFLAGS or CXXFLAGS. Otherwise they won't apply to both C and C++ source
files.

In addition, when using per-target flags, make sure to include the AM_
prefixed variable as well, because otherwise per-target flags will
override it. Conversely, if an assignment sets a per-target flags variable
to only be the AM_ prefixed variable (e.g., "something_CFLAGS =
$(AM_CFLAGS)") then it is redundant.

https://bugzilla.gnome.org/show_bug.cgi?id=761072