tweaks for enabling unity builds

mostly changes to make sure anonymous namespaces don't clash and a change to avoid windows headers from leaking into other compilation units

unity builds are not yet enabled by default, but can be enabled by uncommenting this line in the root `xmake.lua`

```
--add_rules("c++.unity_build")
```

1 files changed, 1 insertions, 0 deletions

diff --git a/xmake.lua b/xmake.lua
index 7b5822371..1b6724044 100644
--- a/xmake.lua
+++ b/xmake.lua
@@ -32,6 +32,7 @@ if not is_arch("arm64") then
 end
 
 add_rules("mode.debug", "mode.release")
+--add_rules("c++.unity_build")
 
 if is_mode("release") then
     set_optimize("smallest")