Simplify ly_symbol2scm (issue 363750043 by address@hidden)

[nine . fierce . ballads]

https://codereview.appspot.com/363750043/diff/1/lily/include/lily-guile-macros.hh
File lily/include/lily-guile-macros.hh (right):

https://codereview.appspot.com/363750043/diff/1/lily/include/lily-guile-macros.hh#newcode95
lily/include/lily-guile-macros.hh:95: value;
                                 \
This is new to me.  It is a GCC extension, according to
https://stackoverflow.com/a/1635559.  I'd probably add a TODO comment
that suggests converting this to a lambda once we can depend on C++11.

https://codereview.appspot.com/363750043/