If no threading library is available, vtkConditionVariable
defines some fake typedefs and functions that look like they
belong to the pthread library but really do nothing.  I ran
into a case while compiling for compute node linux (kind of
like cross compiling except that the target platform supports
a subset of functionality as the compiling node rather than
being a totally different architecture) where some include of
an include was including the pthreads header even though the
build does not support pthreads.  When vtkConditionVariable
tried to make its own pthread-like declarations, the compiler
barfed because there were multiple definitions.  This fix
simply places all these local definitions in a namespace.
This makes the multiple definitions distinguishable by the
compiler and at worst you should get a warning about shadowed
symbols.