Geometry Engine Open Source is a C++ port of the Java Topology Suite released under the LGPL license. It has interfaces for C++, C and python (though swig).
The recommended low-level interface to the GEOS library is the simplified C wrapper interface. This will ensure stability of the API and the ABI of the library during performance improvements that will likely change classes definitions.
If you don't care about adapting/rebuilding your client code you can still use the C++ interface.