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.

Signed-off-by: pmmccorm <patrick.m.mccormick@intel.com>