Roll to v6.0 and update other headers to track

Signed-off-by: Ralph Castain <rhc@pmix.org>

"make check" can only succeed if the plugins have been installed or
"--disable-dlopen" has been passed to configure.  The first can't
happen in "make distcheck", so force the second for all invocations
of "make distcheck" by setting AM_DISTCHECK_CONFIGURE_FLAGS in
Makefile.am.

Clean up the handling of AM_DISTCHECK_CONFIGURE_FLAGS and
DISTCHECK_CONFIGURE_FLAGS in make_dist_tarball.  First, with the
above fix, passing --disable-dlopen is no longer required.  Second,
users (and scripts) should set DISTCHECK_CONFIGURE_FLAGS, not
AM_DISTCHECK_CONFIGURE_FLAGS.  Automake will combine the two options
when running "make distcheck", so the right things will happen with
this configuration.

Signed-off-by: Brian Barrett <bbarrett@amazon.com>

Signed-off-by: Ralph Castain <rhc@pmix.org>