Build: configure: allow building without Pacemaker Remote support

This was always the intent, but it was broken because the executor Makefile
unconditionally tried to build pacemaker-remoted, and because its
execd_commands.c did not guard TLS-specific code with HAVE_GNUTLS_GNUTLS_H.

Now, we only try to build pacemaker-remoted if support is available, so we can
just check the constant defined there (renamed to PCMK__COMPILE_REMOTE) rather
than check both that and HAVE_GNUTLS_GNUTLS_H.

We also newly take remote support into account in the regression tests.