select/cray: update compile-time and runtime support for Cray build

These changes update build support for Cray XT/XE:
 1. renamed '--cray-xt' into '--cray' since also XE systems are supported;
 2. autoconf rules to cover the various possible build cases:
    a) --enable-cray=off: HAVE_CRAY/HAVE_NATIVE_CRAY undefined,
    b) --enable-cray=on:  HAVE_CRAY defined
       b1) local host is a native Cray system: HAVE_NATIVE_CRAY defined
           (requires installation of mysql-devel and libexpat-devel packages),
       b2) local host is not a native Cray system: the conditionally built
           parts (basil_interface.c, libalps.la) are not built;
 3. updated configure logic:
    - since Cray support depends on mySQL, reordered tests in configure.ac,
    - reordered logic with regard to changes in (2),
    - an AM_CONDITIONAL to build native-Cray parts conditionally,
    - updated configure messages (XT/XE);
 4. run-time read_conf test to ensure use of select/cray is properly supported,
 5. an update of the NEWS file due to the change in (1) ==> may have a conflict
    in case you have a locally-updated copy.

I have compile-tested the three possible scenarios in (2).