Forced to take ROOT internal mutex lock to avoid deadlock

ROOT locks the same mutex before loading a shared library and in
statics embedded in shared libaries. In the middle the operating
system takes a lock to protect the shared library data structures.
Therefore to avoid deadlock problems we must take the same ROOT lock
before loading any shared libraries since we do not know apriori
if the shared library being loaded links to a library which will
take the lock at load time.
Merge pull request #2938 from Dr15Jones/takeRootCintLockBeforeLoadingPlugins

Mulitthreading -- Forced to take ROOT internal mutex lock to avoid deadlock