[meson] fix version header custom target for meson >= 0.57

Apparently custom_target no longer supports the @CURRENT_SOURCE_DIR@
and @SOURCE_DIR@ substitutions.
We now locate the script using find_program().
Since meson.project_source_dir() is not available in older meson version
there is a version check.
17 jobs for fix_meson_custom_target in 22 minutes and 48 seconds (queued for 1 second)