Replace deprecated Automake INCLUDES variable with AM_CPPFLAGS

Excerpt https://lists.gnu.org/archive/html/automake/2012-12/msg00038.html



  - Support for the long-deprecated INCLUDES variable will be removed
    altogether in Automake 1.14.  The AM_CPPFLAGS variable should be
    used instead.

This variable was deprecated in Automake releases prior to 1.10, which is
the current minimum level required to build X.

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>