replace ODIN_OS string with enums

vendor/stb/image/stb_image.odin

@@ -4,9 +4,9 @@ import c "core:c/libc"
 
 #assert(size_of(c.int) == size_of(b32))
 
-when ODIN_OS == "windows" { foreign import stbi "../lib/stb_image.lib" }
-when ODIN_OS == "linux"   { foreign import stbi "../lib/stb_image.a"   }
-when ODIN_OS == "darwin"   { foreign import stbi "../lib/darwin/stb_image.a"   }
+when ODIN_OS == .Windows { foreign import stbi "../lib/stb_image.lib" }
+when ODIN_OS == .Linux   { foreign import stbi "../lib/stb_image.a"   }
+when ODIN_OS == .Darwin  { foreign import stbi "../lib/stb_image.a"   }
 
 #assert(size_of(b32) == size_of(c.int))