Makefile.am: Fix an automake warning

Makefile.am: Fix an automake warning

Fix the following automake warning message:

    Makefile.am:1: `INCLUDES' is the old name for `AM_CPPFLAGS' (or `*_CPPFLAGS')

A quote from the automake manual:

    INCLUDES
        This does the same job as AM_CPPFLAGS (or any per-target _CPPFLAGS variable
        if it is used). It is an older name for the same functionality. This
        variable is deprecated; we suggest using AM_CPPFLAGS and per-target
        _CPPFLAGS instead.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>