Make exported symbols check optional

Recently I investigated why sometimes linking of some libraries in my project takes so much time comparing to qmake. I noted something is happening inside QBS after ld disappears from the process list. I found that "something" is exported symbols check. The problem is reproduced on libraries which have too many exported symbols (over 100000). For them QBS takes over than 2 minutes to perform the check. This is much longer than just re-linking all the dependent products (also this is not the only library which have this problem).

So my suggestion is to add another possible value for cpp.exportedSymbolsCheckMode: "skip" or "disabled" which will disable this check for libraries like the described one.