Only when compiling with cmake will sclang-vars.el be written. This
commit moves relevant constants into customizable vars. Only if the
`sclang-system-data-dir` exists is it respected, otherwise it will guess
the proper location by looking at the system-type.
The purpose of this is to make it possible to build and distribute a
package independent of the SuperCollider build process, while ensuring
that those users who have become accustomed to the current install
process are not disrupted.
