haiku support (part 2)

vlib/compiler/main.v

@@ -966,6 +966,9 @@ pub fn new_v(args[]string) &V {
 		$if solaris {
 			_os = .solaris
 		}
+		$if haiku {
+			_os = .haiku
+		}
 	}
 	else {
 		_os = os_from_string(target_os)
@@ -1149,6 +1152,7 @@ pub fn os_from_string(os string) OS {
 			// notice that `-os msvc` became `-cc msvc`
 			verror('use the flag `-cc msvc` to build using msvc')
 		}
+		'haiku' { return .haiku }
 	}
 	println('bad os $os') // todo panic?
 	return .linux