Blender Git Loki

MSVC: Fix macro collision with MSVC Headers

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