Makefile.am: replace deprecated "INCLUDES" with "AM_CPPFLAGS"

This fixes the following warning:

warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')
Merge pull request #5 from Mansouu/patch-1

Makefile.am: replace deprecated "INCLUDES" with "AM_CPPFLAGS"