Use AM_CPPFLAGS since autotools deprecated INCLUDE
autoconf deprecated INCLUDE around version 2.69. No other automake file
in the project uses INCLUDE (instead they use AM_CPPFLAGS) and this
warning prevents the bootstrap-configure-make process from working on
OSX for me. Updating the variable name in the include/Makefile.am fixes
this and allows me to move along with development on OSX.