portab.h fixed for Mac
portab.h fixed for Mac

For some reason nullptr caused problem in some source files, not others, on the Mac with g++.
It seems more general anyway to test for existence than for __clang__.

sorenhein/dds,Afwas/dds,dds-bridge/dds,sorenhein/dds,dds-bridge/dds,Afwas/dds