Picked up while investigating a build error on the functions branch
which seems to use this specific header in a way master doesn't.
The problem:
The MSVC headers define a `_CONCAT` macro, so does BLI_kdtree_imp.h
however at the end `BLI_kdtree_imp.h` undefines the macro making the
MS headers that still rely on it "unhappy".
Who's fault is this:
Ours, C99 Spec says
```
7.1.3 Reserved identifiers
- All identifiers that begin with an underscore and either an uppercase letter or another underscore are always reserved for any use.
...
if the program removes (with #undef) any macro definition of an identifier in the first
group listed above, the behavior is undefined.
```
So we should not have defined it, and we definitely should not
have undefined it.
We have *tons* of these violations, although fixing them would be great
at one point lots of them are in /extern or in the 3rd party deps,
I'd rather deal with them on a case by case basis when it actually
causes issues.
Differential Revision: https://developer.blender.org/D5790
Reviewers: campbellbarton, JacquesLucke
This moves logic into kdtree_impl.h which is included in a source
file that defines the number of dimensions - so we can easily support
different numbers of dimensions as needed
(currently 3D and 4D are supported).
Macro use isn't so nice but avoids a lot of duplicate code.
