Fix warning: use AM_CPPFLAGS instead of INCLUDES

Before the fix, 'autogen.sh' prints the below warning:

	configure.ac:709: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')

Log of 'autogen.sh':

	$ ./autogen.sh
	libtoolize: putting auxiliary files in AC_CONFIG_AUX_DIR, `src/build-aux'.
	libtoolize: copying file `src/build-aux/ltmain.sh'
	libtoolize: putting macros in AC_CONFIG_MACRO_DIR, `src/m4'.
	libtoolize: copying file `src/m4/libtool.m4'
	libtoolize: copying file `src/m4/ltoptions.m4'
	libtoolize: copying file `src/m4/ltsugar.m4'
	libtoolize: copying file `src/m4/ltversion.m4'
	libtoolize: copying file `src/m4/lt~obsolete.m4'
	configure.ac:709: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')
	configure.ac:709: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')

This patch removes the warning.