v6-20-02-alice5

Avoid {using namespace std} in global scope of dictionary [ROOT-10661]

In order to avoid {namespace std} interfering with following include files,
when generating a dictionary file add the {using namespace std;} only after
GenerateNecessaryIncludes method is called.

Since many legacy root header files do require this global {using namespace std;}
to complete the tests, old behaviour is kept by default and the global
{using namespace std} can be avoided by adding -noGlobalUsingStd to rootcling invocation.

Allow passing ACLiC RootCling flags via .rootrc (Vassil)

This is a special patch tailored for v6-20-02