Implement shapefile SBN indexing

[janhec]

I developed a C++ shapefile indexing routine, inspired on ​https://github.com/drwelby/hasbeen. It functions within gdal 1.11.0 at the level of being callable through shapefil.h. Integration in the driver is still missing. Since QIX indexing is now the format for index creation, an additional SQL like keyword is needed. Suggestion is "CREATE SPATIAL INDEX ON foo ALGORITHM SBN". I need assistance completing this step. Please mail me at jan.heckman AT gmail.com if you are willing to undertake this part of the work. I will supply SBNIndex.cpp and two slight mods of shapefil.h and sbnsearch.c. SBNIndex is callable from C and has been compiled and tested with VS2013 and VS2003, to test compatibility.