Separate UTF-8 flags by platform (#178)

Fixes the following warnings
```
1>cl : Command line warning D9002: ignoring unknown option '-source-charset=utf-8'
1>cl : Command line warning D9002: ignoring unknown option '-execution-charset=utf-8'
```

1 files changed, 5 insertions, 2 deletions

diff --git a/xmake.lua b/xmake.lua
index 4d04506c6..a8256dbd5 100644
--- a/xmake.lua
+++ b/xmake.lua
@@ -67,8 +67,11 @@ else
 end
 
 -- fmt 11+ requires utf-8 when using unicode
-set_encodings("utf-8")
-set_encodings("source:utf-8", "target:utf-8")
+if is_os("windows") then
+    set_encodings("utf-8")
+else
+    set_encodings("source:utf-8", "target:utf-8")
+end
 
 if is_os("windows") then
     add_defines(