Syntax error in ranged tempo specifications

[Joachim Mairböck]

The syntax of a ranged tempo specification changed from \tempo <dur> = <low>
~ <high> to \tempo <dur> = <low> - <high> from version 2.16 to current
unstable, i.e. it uses a '-' instead of a '~'.

This is a good change. However, a source file containing a statement like
this and \version "2.16.0" doesn't compile using the current unstable
version. The \version command should enable compatibility with older
versions (and Lilypond should accept both forms if such a \version statement
exists).