Use Clang as the compiler for the Mac platform.
Changed all Mac to use clang/clang++ as the default C and CXX compilers if
not provided by the user. Still use gfortran as the default fortran
compiler since Apple does not provide any fortran compiler.
HDFFV-8245: bug fix. Snow Leopard (darwin 10.X) does not have clang++.
Reversed it to use gcc/g++ as the default C and CXX compilers for Darwin 10.X
systems.
Tested: h5committest plus duck (darwin 11), wren (darwin 12) and fred
(darwin 10).
82869120a9