Rename INCLUDES to AM_CPPFLAGS

Fixes aclocal 1.13.3 warning:
'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')
Also tested with aclocal 1.11.1

git-svn-id: http://svn.osgeo.org/geos/trunk@3973 5242fede-7e19-0410-aef8-94bd7d2200fb