[RFC] Support --datarootdir

Describe the bug

Meson does not support --datarootdir.

Meson only support --datadir and not --datarootdir, while often these are redundant, there are relatively common cases where this is important.

For example the emulator snes9x uses --datadir to install the desktop file, icons and locales, on Slackware these must be installed to /usr/share/applications, /usr/share/icons and /usr/share/locale respectively. However they also install some data in the /usr/share/snes9x directory which would be preferable to install to /usr/share/games/snes9x instead.

Before when snes9x used autotools I could leave --datarootdir to the default of /usr/share and set --datadir=/usr/share/games to get the intended behavior. Now snes9x has worked around this by adding their own appdatadir option where I set -Dappdatadir=share/games/snes9x. Suffice to say this is less than ideal and it should not have to be added to every applicable project one by one.

https://github.com/snes9xgit/snes9x

Expected behavior

Meson should support --datarootdir and not force projects to reinvent the wheel every time its applicable.

system parameters

OS: Slackware64-current
python: 3.8.1
meson: 0.55.1

No file or test is named in the issue. Start by tracing Meson's existing --datadir command-line handling and how installation data paths are derived; done means --datarootdir works without project-specific appdatadir workarounds while preserving current behavior.