scope_guard: fix silly mistake in ON_SCOPE_EXIT.

The previous implementation worked when used once per function, which
happened to be enough to pass the tests.  Macro token pasting is now
fixed.

concatenate.h

@@ -0,0 +1,11 @@
+#ifndef _ETL_CONCATENATE_H_INCLUDED
+#define _ETL_CONCATENATE_H_INCLUDED
+
+/*
+ * A macro for concatenating text tokens.  Doing this is surprisingly involved
+ * with the C preprocessor.
+ */
+#define ETL_CONCATENATE(a, b) _ETL_CONCATENATE(a, b)
+#define _ETL_CONCATENATE(a, b) a ## b
+
+#endif  // _ETL_CONCATENATE_H_INCLUDED

scope_guard.h

@@ -3,6 +3,7 @@
 
 #include <type_traits>
 
+#include "etl/concatenate.h"
 #include "etl/implicit.h"
 #include "etl/utility.h"
 
@@ -211,7 +212,7 @@ ScopeGuard<typename std::decay<F>::type> operator<<(GuardTag const &,
 }
 
 #define ETL_ON_SCOPE_EXIT \
-  auto on_scope_exit_ ## __COUNTER__ = ::etl::GuardTag() << [&]()
+  auto ETL_CONCATENATE(on_scope_exit, __COUNTER__) = ::etl::GuardTag() << [&]()
 
 }  // namespace etl