hdf5_ep

[svn-r23434] 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.

Tested: passed all three macs OS, snow leapord (fred), lion (duck) and
mountain lion (wren). Also passed test using Apple gcc, Gnu gcc, Intel
as the C compilers and all corresponding CXX compilers fits in.
Did not run h5committest or other non-mac platform since this change
affect Apple platforms only.