maint: Support running `./bootstrap' from a tarball.

This fixes VERSION being empty in ./configure, which may to documentation
having empty version strings.

* Makefile.am (EXTRA_DIST): Add build-aux/git-version-gen.

Change-Id: If127519811b25e2df0f5caa6a83a4f860fd34eb2

Author: janneke

Makefile.am

@@ -724,6 +724,7 @@ EXTRA_DIST +=						\
   build-aux/config.rpath				\
   build-aux/convert-xref.scm				\
   build-aux/generate-authors.scm			\
+  build-aux/git-version-gen				\
   build-aux/test-driver.scm				\
   build-aux/update-NEWS.scm				\
   build-aux/update-guix-package.scm			\