`make bytecode && make doc` broken

[Werner LEMBERG]

[b30a94a7f8da18a8b08d3c4f11bb95c278a83c57]

Currently, `make bytecode && make doc` is broken:

```
Command '/home/wl/build-lilypond/out/bin/lilypond [...] \
        ".../out/lybook-db/snippet-names-c727b9067d4d978254fae040dfb82c3e.ly"'
        returned non-zero exit status 1.
```

In `snippet-names-c727b9067d4d978254fae040dfb82c3e.ly`:

```
Processing `69/lily-2e7a024b.ly'
Parsing...
Renaming input to: `.../display-lily-tests.ly'
Test unequal: BUG.
input    = { \slashedGrace c8 d2 }
expected = { \slashedGrace c8 d2 }
actual   = { \grace { \initialContextFrom c8 { \override Flag.stroke-style = 
"grace"
      } c8 { \revert Flag.stroke-style
      } } d2 }
...display-lily-tests.ly:252:8: fatal error:
  Guile signaled an error for the expression beginning here
\test #
       #[ \key e \minor #]
```

Apparently this wasn't cought by our CI process.  I thus wonder
whether we should change the invocation to use `bytecode` as a build
target, either for testing, or for the documentation, or for both.


    Werner