git-version-gen: use a reasonable fallback for dist tarballs

Since commit 3.11.0-36-g8612d1c logrotate reported UNKNOWN version
in case someone ran autoreconf in the directory extracted out of the
tarball produced by 'make dist'.

This commit makes autoreconf pick the version string from the suffix
of the directory where build-aux/git-version-gen runs (usually the
top-level directory).  This works reliably for sources extracted out
of the tarballs produced by 'make dist'.

Bug: https://bugzilla.redhat.com/1500264

build-aux/git-version-gen

@@ -189,7 +189,8 @@ then
     v=`echo "$v" | sed 's/-/./;s/\(.*\)-g/\1-/'`;
     v_from_git=1
 elif test "x$fallback" = x || git --version >/dev/null 2>&1; then
-    v=UNKNOWN
+    # fallback for tarballs produced by 'make dist' (use directory suffix)
+    v="`basename "$PWD" | sed 's/^[^-]*-//'`"
 else
     v=$fallback
 fi