Revert "C++17: switch the default."

This reverts commit c35fc0e1306ca471063f569f67f4bc6c77ba3c0d.

Reason for revert: fix broken targets in the internal master.

Change-Id: I70447a05ae5b2afa7efc7ab21c0432e651cab1ba
diff --git a/cc/config/global.go b/cc/config/global.go
index ef9ac80..8fd5914 100644
--- a/cc/config/global.go
+++ b/cc/config/global.go
@@ -114,7 +114,7 @@
 	}
 
 	CStdVersion               = "gnu99"
-	CppStdVersion             = "gnu++17"
+	CppStdVersion             = "gnu++14"
 	ExperimentalCStdVersion   = "gnu11"
 	ExperimentalCppStdVersion = "gnu++2a"