Macro rename and move them outside for global usage; Tests extended to other dimensions

[AIG-Livny]

Hi! This is a very cool library! Thank you very much! I would like to use it in my pet project and have made some minor improvements:

• I found it useful to move macros outside. It can be set globally or before rtree.h including. Also there is default values in rtree.h.
• Macro renamed - added prefix "RTREE_" for global setup
• All tests extended to other dimensions and number types of spatial coords
• don’t know if this is the right thing, but I removed priv_funcs.h because to access private functions in tests, can be included the entire rtree.c
• changed macro bench to move code into functions and being able to look at it with a debugger
• It may appear that the is_float_point macro is running runtime, but compiler will optimize it
• I wanted to keep the original code in the tests for dimension 2 and type double because longitude and latitude are used there, obviously this is a more common case of application. So now there is a macro #if RTREE_DIMS == 2
• macro DISABLE_WARNING got from https://www.fluentcpp.com/2019/08/30/how-to-disable-a-warning-in-cpp/

recreated PR, because mistake the branch

[logankaser]

It would be great to merge this, even just for the:

#ifdef __cplusplus
extern "C" {
#endif

Change, I patch this library to add that right now.
The un-namespaced macros are a bit collision prone as well