[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.
fix_meson_custom_target in 22 minutes and 48 seconds (queued for 1 second)17 jobs for