Allow C-API users to hide non-reentrant section

Define GEOS_USE_ONLY_R_API to obtain a compile-time error when
trying to use non-reentrant functions.

Patch by Even Rouault (#695)

git-svn-id: http://svn.osgeo.org/geos/trunk@3991 5242fede-7e19-0410-aef8-94bd7d2200fb