meson: de-duplicate binaries using symlinks

[floppym]

Use custom targets to create symlinks for testing.
meson will dereference these at install time, so use install_symlink().

[floppym]

This is a possible alternative to #712.

I personally prefer the end result given by using hard links, but this symlink approach seems a bit cleaner from a meson perspective.

[navi-desu]

the custom target shouldn't be necessary, as the argument for install_symlink is a str, so just

foreach exec: einfo_execs
  install_symlink(exec, install_dir: rc_bindir, pointing_to: 'einfo')
endforeach

works

[floppym]

the custom target shouldn't be necessary, as the argument for install_symlink is a str, so just

The custom targets are to populate symlinks in the build directory for use by the test suite.

[navi-desu]

mind if i tweak this to install do_{value,service,mark} instead, and symlink to that? (already done it, just wanted to get confirmation)

[floppym]

If you want to make that adjustment, I don't mind.

Just reset the author to yourself because it wasn't my idea and I prefer not to have non-functional binaries tied to my name.