Update conditional-compilation.md

Mention the "none" target_os value

Author: evanperrygrove-somewear

src/conditional-compilation.md

@@ -129,6 +129,7 @@ Example values:
 * `"dragonfly"`
 * `"openbsd"`
 * `"netbsd"`
+* `"none"` (typical for embedded targets)
 
 ### `target_family`