[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)
Status Name Job ID Coverage
  Smoke Test
passed smoke-test #318039

00:16:56

passed static-analysis-with-emper-io #318040

00:16:32

 
  Test
passed test-clang #318044

00:02:04

passed test-clang-debug #318046

00:01:40

passed test-clang-release #318045

00:01:40

passed test-clang-sanitizer-undefined #318050

00:01:36

passed test-do-not-log-timestamp #318054

00:05:48

passed test-gcc #318041

00:02:36

passed test-gcc-debug #318043

00:02:34

passed test-gcc-release #318042

00:02:04

passed test-laws #318051

00:03:04

passed test-laws-release #318052

00:01:28

passed test-locked-ws-queues #318055

00:05:48

passed test-with-stats #318048

00:02:34

passed test-with-userspace-rcu #318049

00:03:06

passed test-worker-no-sleep #318047

00:02:32

passed test-worker-wakeup-strategy-all #318053

00:03:14