build: fix automake warning about preprocessor C flags

With automake version 1.13.2 (which comes in debian testing/jessie),
we were starting to get these warnings by default:

...
Running automake --foreign  ...
atk/glue/Makefile.am:16: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')
gio/glue/Makefile.am:11: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')
gtk/glue/Makefile.am:18: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')
gtk/gui-thread-check/profiler/Makefile.am:8: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')
pango/glue/Makefile.am:13: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')
sample/opaquetest/Makefile.am:18: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')
sample/valtest/Makefile.am:18: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')
Running autoconf ...
...

We simply follow the warning's recommendation of using AM_CPPFLAGS instead
(CPP meaning C PreProcessor, not C Plus Plus), as explained in
http://www.gnu.org/software/automake/manual/html_node/Program-Variables.html

The deprecation of INCLUDES has been very long there already (since 2002,
therefore Automake 1.7), and we already depend on automake 1.10.
Merge pull request #62 from knocte/automake_warnings

build: fix automake warning about preprocessor C flags