Update Makefile.am

Resolve an error with autotools that looked like:

    Makefile.am:22: error: cgminer_CPPFLAGS must be set with '=' before using '+='
    Makefile.am:13: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')
    Makefile.am: installing './depcomp'
    autoreconf: automake failed with exit status: 1