meson: add options to override build information

We inject various different kinds of build information into build
artifacts, like the version string or the commit from which Git was
built. Add options to let users explicitly override this information
with Meson.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/meson.build

@@ -219,6 +219,7 @@ asciidoc_conf = custom_target(
   input: meson.current_source_dir() / 'asciidoc.conf.in',
   output: 'asciidoc.conf',
   depends: [git_version_file],
+  env: version_gen_environment,
 )
 
 asciidoc_common_options = [