This lets a packager ensure that the binary object built from an
unpacked source tarball is identical to the binary object created from
a live git tree.

I hope to use it in debian with something like -Drevno=debian/3.36.1-1
during the meson configuration, so that the help dialog box and the
bug reporting logs can identify the specific build.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>