flux plugin build and install broken

[grondo]

It appears that in c3ef9a2f242b36808da92e2ef613bb80299e6f4f some of the autotools setup for the Spindle Flux plugin was removed.

Unfortunately, this breaks proper (or perhaps more correctly _convenient) installation of the Flux plugin. Things that would help the install of the flux plugin include:

• It is customary for plugins to not have a lib prefix, e.g. the Flux plugin should be spindle.so. This matches how Flux expects plugins to be named, but isn't critical if there is a reason Spindle wants the lib prefix.
• The plugin should be installed by default to the Flux shell pluginpath (this was previously captured and defined in configure.ac). For a spack package or side install, a flux shell spindle.lua rc file should be installed and added to FLUX_SHELL_RC_PATH.

I'm also not seeing any checks for libflux-core.so and libflux-hostlist.so, but I must be missing this because the flux plugin is built with these libs and FLUX_CORE_LIBS and FLUX_CORE_CFLAGS etc are defined in the Makefiles, so I apologize if I missed something obvious.