The issue manifests when sclang is compiled. Steps to reproduce in real
emacs config:
1. Install the package and compile it
2. Open a file with a `.sc` or `.scd` extension
3. Witness the error `(void-function sclang-make-buffer-name)`
Steps to reproduce with the test:
0. install [eldev](https://github.com/doublep/eldev#installation)
1. `eldev compile`
2. `eldev test`
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.
