libmcpcad test build (BRL-CAD 7.43.0, Linux x86_64)

A test build of the libmcpcad branch, so you can try the MCP interface without
building BRL-CAD first. Linux x86_64.

libmcpcad gives BRL-CAD a loopback socket, so another program can run geometry
commands against a model you have open. In MGED that is the mcp_listen
command, and the view refreshes as commands come in.

Proposed upstream as PR #238.

Use it

tar xJf BRL-CAD_7.43.0_linux_x86_64_libmcpcad.tar.xz
cd brlcad-7.43.0-libmcpcad

cp /path/to/model.g work.g          # it edits the file you give it
./bin/mged work.g

Then at the MGED prompt:

mcp_listen 5555

MGED stays usable while it listens. mcp_listen status reports the state and
mcp_listen off stops it.

If you have no GUI, or you are on a remote machine, there is a headless listener
instead:

./bin/mcpcad_test_server 5555 work.g

Either way, point the agent at port 5555:
brlcad-mcp rc1.

Notes

• Needs glibc 2.34 or newer, so Ubuntu 22.04+, Fedora 35+, or RHEL 9+.
• The MGED GUI uses X11, fontconfig, freetype, harfbuzz and libXScrnSaver from
  your distro. On Fedora, sudo dnf install libXScrnSaver. The headless
  listener, rt and nirt do not need them.
• Built with debug symbols, so renders are slower than a release build.
• 7.43.0 is a development version under BRL-CAD's numbering, not a BRL-CAD
  release. Those come from BRL-CAD/brlcad.