Debundle Valgrind and make it optional

If a *nix user wishes to build `pyrogenesis` with support for Valgrind
(such as
it is), then can do so by passing `--with-valgrind` to
`update-workspaces.sh`.
(They will need Valgrind installed and locateable via `pkg-config`.)


Reviewed By: sera, Stan
Fixes: #2904
Differential Revision: https://code.wildfiregames.com/D3646
This was SVN commit r25933.

source/scriptinterface/ScriptExceptions.cpp

@@ -89,10 +89,6 @@ bool ScriptException::CatchPending(const ScriptRequest& rq)
 	}
 
 	LOGERROR("%s", msg.str().c_str());
-
-	// When running under Valgrind, print more information in the error message
-	//	VALGRIND_PRINTF_BACKTRACE("->");
-
 	return true;
 }