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

source/blender/blenlib/BLI_kdtree_impl.h

@@ -21,9 +21,9 @@
 
 #include "BLI_compiler_attrs.h"
 
-#define _CONCAT_AUX(MACRO_ARG1, MACRO_ARG2) MACRO_ARG1##MACRO_ARG2
-#define _CONCAT(MACRO_ARG1, MACRO_ARG2) _CONCAT_AUX(MACRO_ARG1, MACRO_ARG2)
-#define BLI_kdtree_nd_(id) _CONCAT(KDTREE_PREFIX_ID, _##id)
+#define _BLI_CONCAT_AUX(MACRO_ARG1, MACRO_ARG2) MACRO_ARG1##MACRO_ARG2
+#define _BLI_CONCAT(MACRO_ARG1, MACRO_ARG2) _BLI_CONCAT_AUX(MACRO_ARG1, MACRO_ARG2)
+#define BLI_kdtree_nd_(id) _BLI_CONCAT(KDTREE_PREFIX_ID, _##id)
 
 struct KDTree;
 typedef struct KDTree KDTree;
@@ -93,6 +93,6 @@ int BLI_kdtree_nd_(range_search_with_len_squared_cb)(
                        const void *user_data),
     const void *user_data) ATTR_NONNULL(1, 2) ATTR_WARN_UNUSED_RESULT;
 
-#undef _CONCAT_AUX
-#undef _CONCAT
+#undef _BLI_CONCAT_AUX
+#undef _BLI_CONCAT
 #undef BLI_kdtree_nd_