hdf5_ep

[svn-r23610] Improvement: DFFV-8245
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).