Covers OS detection, CPU architecture, bitness, and compiler family.
The goal of this change is to provide easier to use and remember checks
for these things. For example, with this change code like
```
#ifdef _WIN32
...
#elif defined(__APPLE__) || defined(__FreeBSD__) || defined(__NetBSD__) || \
defined(__OpenBSD__)
..
#endif
```
becomes
```
#if OS_WIN
...
#elif OS_MAC || OS_BSD
...
#endif
```
The code is originally based on build_config.h from Chromium, which was
first modified for Libmv, then to some other projects, and now is
adopted for Blender itself.
The checks are relying on the -Wundef to provide hint of cases when an
include is missing prior to the platform-specific checks.
This change only introduces possibility of cleaner checks and does not
start actual refactor.
Pull Request: https://projects.blender.org/blender/blender/pulls/118908
baf9691959