You can set the language feature for each project separately by going to Properties => Build tab => Advanced button => Language Version and set your preferred version.
You should realize that it will still use the new “C# 6.0” .Net Compiler Platform (codenamed Roslyn). However, that compiler will imitate the behavior of older compilers and will limit you to features only available on that specific language version.
I don’t think that there’s a solution-wide setting available.