Don't use GTEST_OS_LINUX on Darwin

Change-Id: If77118591daa7b474e6989e03109ea32ddd357b2
diff --git a/cc/cc.go b/cc/cc.go
index 25e7147..d2f0930 100644
--- a/cc/cc.go
+++ b/cc/cc.go
@@ -2051,11 +2051,15 @@
 	if ctx.Host() {
 		flags.CFlags = append(flags.CFlags, "-O0", "-g")
 
-		if ctx.HostType() == common.Windows {
+		switch ctx.HostType() {
+		case common.Windows:
 			flags.CFlags = append(flags.CFlags, "-DGTEST_OS_WINDOWS")
-		} else {
+		case common.Linux:
 			flags.CFlags = append(flags.CFlags, "-DGTEST_OS_LINUX")
 			flags.LdFlags = append(flags.LdFlags, "-lpthread")
+		case common.Darwin:
+			flags.CFlags = append(flags.CFlags, "-DGTEST_OS_MAC")
+			flags.LdFlags = append(flags.LdFlags, "-lpthread")
 		}
 	} else {
 		flags.CFlags = append(flags.CFlags, "-DGTEST_OS_LINUX_ANDROID")