Clean up configure output, don't do unused checks.

Remove unused checks: \*_CLOEXEC, AC_C_CONST, sizeof long.
Print which providers and how they are built after configure runs, don't error when no provider
is built: libfabric.so could be used with only out-of-tree providers. Print a big warning instead
and return 1. Output at end of configure looks like:

    ...
    checking for linux/netlink.h... yes
    checking for nl_connect in -lnl... no
    configure: usnic provider disabled
    checking that generated files are newer than configure... done
    configure: creating ./config.status
    config.status: creating Makefile
    config.status: creating libfabric.spec
    config.status: creating config.h
    config.status: config.h is unchanged
    config.status: executing depfiles commands
    config.status: executing libtool commands
    ***
    *** Built-in providers: sockets verbs
    *** DSO providers:
    ***

@jsquyres Does this look sane?

Signed-off-by: pmmccorm <patrick.m.mccormick@intel.com>
To be clear: the above comment about "build up $list_of_providers" is my opinion; I'm not stating that you have to do that.  I personally think it's a little cleaner, but you may well disagree/not care.