Include execinfo.h, cxxabi.h, and dlfcn.h under the same conditions under which we use the APIs from them. Move their inclusion out of OS-specific blocks.
KWSys provides a platform-independent API to many common system features that are implemented differently on every platform. This library is intended to be shared among many projects, so it has a configurable namespace. Each project should configure KWSys to use a namespace unique to itself. See comments in CMakeLists.txt for details. You are probably reading this file in the source tree of a surrounding project. In that case, see "../README.kwsys" for details of using KWSys in your project.