Install but do not distribute generated headers

These are platform.h and version.h
Fixes bug #601 and the lack of C++ headers install in
releases 3.4.0 and 3.4.1

git-svn-id: http://svn.osgeo.org/geos/trunk@3896 5242fede-7e19-0410-aef8-94bd7d2200fb