[BUG?] org-babel-lilypond-commands can't be set through customize

[Victor A. Stoichita]

Hi!

In ob-lilypond the variable org-babel-lilypond-commands appears to 
be in a weird state. The customize interface says:
org-babel-lilypond-commands: nil
  > [ State ]: UNKNOWN, you should not see this.
  > Commands to run lilypond and view or play the results. 
  > These should be executables that take a filename as 
  > an argument.
  > On some system it is possible to specify the filename 
  > directly
  > and the viewer or player will be determined from the file 
  > type;
  > you can leave the string empty on this case.
[C-h v] says:
org-babel-lilypond-commands is a variable defined in 
‘ob-lilypond.el’.
It is void as a variable.

I can't set the variable through customize. I don't know what the 
syntax would be to set it with setq.

Actually I only need to customize the more specific 
org-babel-lilypond-midi-command. I can set this one with something 
like
(setq org-babel-lilypond-midi-command "timidiy").

However its docstring says:
org-babel-lilypond-midi-command is a variable defined in 
‘ob-lilypond.el’.
Its value is "timidity"

  This variable may be risky if used as a file-local variable.

Documentation:
Command to play a MIDI file on your system.
Do not set it directly. 
Customize ‘org-babel-lilypond-commands’ instead.

Hence my attempt to customize org-babel-lilypond-commands.

I have org-mode 9.3.1. 

Regards,
Victor