source_root function is deprecated in Meson version 0.56.0, so let's use instead a current_source_dir() function, available in all Meson versions. This also allows to deduplicate some code by declaring commonly used string at the top meson.build file. Signed-off-by: Konstantin Kharlamov <Hi-Angel@yandex.ru> Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>. Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17974>