This has never, ever, ever worked. You can get away with it a tiny, tiny
bit, iff you magically assume several things about both internal
implementations, as well as the project source layout and builddir
location.
This can be witnessed by the way using files() was mercilessly tortured
through joining the undefined stringified format value of the file to
the current source dir... because it didn't actually *work*, the
stringified value isn't an absolute path or a builddir-relative one, but
it works as long as you do it from the root meson.build file.
Furthermore, this triggers a deprecation warning if you do it. And using
it for files() is frivolous, the "static map file" case was correct all
along.
Fix the configure_file case to demonstrate the same painful hoops we
must jump through to get custom_target outputs to work correctly.
Fixes#12259
c3fe1b046d