Keep a copy of CheckCXXSymbolExists.
Keep a copy of CheckCXXSymbolExists.

It seems this was only introduced with CMake 2.8.6.

Change-Id: I788e5a51249699fb0bd7aa8c231728cab4b478ab

ghutchis/avogadrolibs,wadejong/avogadrolibs,cjh1/mongochemweb-avogadrolibs,OpenChemistry/avogadrolibs,ghutchis/avogadrolibs,cjh1/mongochemweb-avogadrolibs,cjh1/mongochemweb-avogadrolibs,OpenChemistry/avogadrolibs,qust113/molcore,ghutchis/avogadrolibs,OpenChemistry/avogadrolibs,ghutchis/avogadrolibs,qust113/molcore,wadejong/avogadrolibs,cjh1/mongochemweb-avogadrolibs,OpenChemistry/avogadrolibs,wadejong/avogadrolibs,ghutchis/avogadrolibs,OpenChemistry/avogadrolibs,wadejong/avogadrolibs