build-support: Added makeSymlinks function

I have refractored things a little bit. We need a function that would symlink
a bunch of files (ex. created with writeText) to a folder. What `environemnt.etc`
uses looks like a fit, because it has really nice syntax where you can specify
`source`, `target` and `mode` so i generalize its functionality. I also added
support for providing only derivation in a list of files to symlink
where name is used as a `target` (usefull in a case if you use `writeText`).