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>
Minor preference: I know you're printing the list at the bottom (which is good), but I don't mind the AC_MSG_NOTICE in the middle of the output, too.

In the OMPI project, we've found that there are definitely users who will look for output in the middle of configure, and there are users who will only look at the bottom of the configure output.

I won't fight to keep these AC_MSG_NOTICEs, but I just felt like I should mention that I certainly don't mind them, and find them genuinely useful when I don't want to have to scroll all the way down to the bottom of the configure output.  :-)