Because meson does not provide a simple way to get the relative build
path, it is computed with a pair of foreach loops. As meson does not
have a simple way to compute string length (except via underscorify
and 63 split operations), the build script uses a shell command
instead.
If the compiler does not suppot -fmacro-prefix-map, then fall back
to passing in the relative path prefix, and use its length to offset
the uses of __FILE__ in log messages so that the build path is at
least still not included in the logs. This is significantly more
efficient than calling _sway_strip_path.
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