In particular this makes them more MinGW-friendly: pkg-config on Windows
has specific code to rewrite the ${prefix} when installed in a
different prefix.

[add @datarootdir@, expand commit message -smcv]
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=75858