add clang/clang++ to compiler search lists

At Pavan's request, this is at a lower priority than gcc.  Note,
however, that clang might still be selected by default on some platforms
if it is aliased to `cc`/`c++`, which are preferred over `gcc`/`g++` as
of [1a1ff11e].

No reviewer.
Update CHANGES file about compiler reordering.

Added a warning in the CHANGES file about the potential ABI breakage
because of the compiler reordering if CC and friends are not added.

Reviewed by goodell.
Fix for #1813

Pass the second argument to MPI_WIN_CREATE in Fortran as an address-sized
integer to fix a test failure.