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

Florian Fischerrequested to merge
aj46ezos/emper:fix_meson_custom_target into master

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.

Edited by Florian Fischer

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