5_1-new: Fix building with clang/clang++ (#1866)

* Have clang and libc++ working.

If any clang users run into problems, remember to have libc++ and libc++abi set up.
Fixes
stepmania/src/arch/ArchHooks/ArchHooks.h:37:27: error:
      variable has incomplete type 'tm'
        virtual void SetTime( tm ) { }
                                 ^
/usr/include/wchar.h:83:8: note: forward declaration of 'tm'
struct tm;
       ^

* Add noexcept to swap.

See https://github.com/stepmania/stepmania/commit/e860773799f01d02f281c8c39421e7e34658788d#commitcomment-32674320

* Use standard c++11 for handling unexpected exceptions

The cxxabi.h header can make linking difficult.

src/archutils/Unix/AssertionHandler.cpp

@@ -46,18 +46,28 @@ extern "C" void __assert_perror_fail( int errnum, const char *file, unsigned int
 
 /* Catch unhandled C++ exceptions.  Note that this works in g++ even with -fno-exceptions, in
  * which case it'll be called if any exceptions are thrown at all. */
-#include <cxxabi.h>
 void UnexpectedExceptionHandler()
 {
-	type_info *pException = abi::__cxa_current_exception_type();
-	char const *pName = pException->name();
-	int iStatus = -1;
-	char *pDem = abi::__cxa_demangle( pName, 0, 0, &iStatus );
-
-	const RString error = ssprintf("Unhandled exception: %s", iStatus? pName:pDem);
+	std::exception_ptr exptr = std::current_exception();
+	try
+	{
+		std::rethrow_exception(exptr);
+	}
+	catch (std::exception &ex)
+	{
 #if defined(CRASH_HANDLER)
-	sm_crash( error );
+		const RString error = ssprintf("Unhandled exception: %s", ex.what());
+		sm_crash( error );
 #endif
+	}
+	// TODO: Don't throw anything not subclassing std::exception
+	catch(...)
+	{
+#if defined(CRASH_HANDLER)
+		const RString error = ssprintf("Unknown exception.");
+		sm_crash( error );
+#endif
+	}
 }
 
 void InstallExceptionHandler()
@@ -68,7 +78,7 @@ void InstallExceptionHandler()
 /*
  * (c) 2003-2004 Glenn Maynard
  * All rights reserved.
- * 
+ *
  * Permission is hereby granted, free of charge, to any person obtaining a
  * copy of this software and associated documentation files (the
  * "Software"), to deal in the Software without restriction, including
@@ -78,7 +88,7 @@ void InstallExceptionHandler()
  * copyright notice(s) and this permission notice appear in all copies of
  * the Software and that both the above copyright notice(s) and this
  * permission notice appear in supporting documentation.
- * 
+ *
  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
  * OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
  * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT OF