Make gCintMutex and gROOTMutex use the same mutex

In order to avoid deadlocking, the variables gCintMutex and
gROOTMutex now point to the same underlying mutex. The mutex is
also created when TThread::Init() is first called rather than doing
it lazily.
Merge pull request #7 from Dr15Jones/unifyCintROOTMutex

Make gCintMutex and gROOTMutex use the same mutex