Keep bison-generated files in sync.

Keep bison-generated files in sync.

Use a single rule to build header and source files from
the syntax file, thus avoiding any possibility to have
`make' rebuild one but not the other, which might happen
if the syntax and target files have almost the same but
slightly differing timestamps.

See http://www.mail-archive.com/bug-lilypond@gnu.org/msg37081.html

Fixes issue 3528: Keep bison-generated files in sync.
http://code.google.com/p/lilypond/issues/detail?id=3528