Issue 5720: Fix C++11 option (issue 579270051 by trueroad@gmail.com)

[lemzwerg--- via Discussions on LilyPond development]

Mhmm, wouldn't it be better to simply define `M_PI` if it is not
defined?  GNU options might not be available with other compilers...

https://codereview.appspot.com/579270051/