Fix CI error / warnings in SMPTools code

Fix multiple warnings and build errors:

• Mac clang error with c++11 (error cdash) fix here
• Mac clang warning with forward declaration of template (error cdash)
• Mac clang warning loop variable is always a copy (error cdash in TestSMP.cxx)
• Fix warning C4251 for vtkSMPThreadPool.h (error cdash)