configure.ac: INCLUDES to AM_CPPFLAGS

to avoid:
$ ./autogen.sh
configure.ac:712: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')
configure.ac:712: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')
configure.ac:712: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')
configure.ac:712: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')
configure.ac:712: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')

http://www.gnu.org/software/automake/manual/html_node/Program-Variables.html
INCLUDES: This variable is deprecated; we suggest using AM_CPPFLAGS and per-target _CPPFLAGS instead.