First set of changes for a more robust build.

- define macros WITH_GLUT and WITH_CUDA_SDK if GLUT and the CUDA SDK are found
- conditionally include GL code, depending on whether or not WITH_GLUT is defined
- conditionally include the local helper_math.h is WITH_CUDA_SDK is not defined
- add export/import macro definitions (not used yet)
Split library files and model files in different directories.
Add the new files in their respective directories.