Conditional compilation of SQL Server stored procedure
I'm not aware of this being possible with SSDT unfortunately. 

Depending on how big the project is and how many procedures you intend to enhance with 2012 goodies, it may be manageable with Composite Projects.


  SSDT can combine a database project with one or more referenced
  database projects or dacpacs to describe a single composite database
  schema.   Using a composite project allows a large database to be
  broken down into more manageable chunks, allows different people or
  teams to have responsibility for different parts of the overall
  schema, and enables reuse of database object definitions in multiple
  databases.


The notion would be to have a base project, containing the common object definitions and version specific projects for procedures that used new features. The 2012 project would reference the base project and a compile/build would combine objects from both. 

The PITA would be that you can't override an object in the base project with an object in a composite, so you would have to maintain base, 2008 &amp; 2012 projects. When you wanted a 2012 version of a particular procedure, you would have to remove it from base and create a version in both 2008 &amp; 2012 projects.