Include AM_CPPFLAGS in target-specific CPPFLAGS

Attempt to fix build on travis (with automake 1.11.3)
See http://travis-ci.org/libgeos/libgeos/jobs/18602776#L2519

git-svn-id: http://svn.osgeo.org/geos/trunk@3974 5242fede-7e19-0410-aef8-94bd7d2200fb