* String interpolation. Expressions like

"--with-freetype2-library=" + freetype + "/lib"

  can now be written as

    "--with-freetype2-library=${freetype}/lib"

  An arbitrary expression can be enclosed within ${...}, not just
  identifiers.

* Escaping in string literals: \n, \r, \t interpreted as in C, any
  other character following \ is interpreted as-is.
  
* Newlines are now allowed in string literals.

src/libexpr/nixexpr-ast.def

@@ -19,6 +19,7 @@ SubPath | Expr Expr | Expr |
 OpHasAttr | Expr string | Expr |
 OpPlus | Expr Expr | Expr |
 OpConcat | Expr Expr | Expr |
+ConcatStrings | ATermList | Expr |
 Call | Expr Expr | Expr |
 Select | Expr string | Expr |
 Var | string | Expr |