Add support for jailbreak-cabal utility to the Cabal builder.

Jailbreaks-cabal allows Nixpkgs maintainers to quick-fix builds of packages
that over-specify their version requirements by removing the version
restrictions of all dependencies from the Cabal file. Set

  jailbreak = true

in the build expression to activate this feature.

pkgs/build-support/cabal/default.nix

@@ -1,6 +1,6 @@
 # generic builder for Cabal packages
 
-{stdenv, fetchurl, lib, pkgconfig, ghc, Cabal, enableLibraryProfiling ? false} :
+{ stdenv, fetchurl, lib, pkgconfig, ghc, Cabal, jailbreakCabal, enableLibraryProfiling ? false }:
 {
   mkDerivation =
     args : # arguments for the individual package, can modify the defaults
@@ -86,7 +86,8 @@
             configurePhase = ''
               eval "$preConfigure"
 
-              for i in Setup.hs Setup.lhs; do
+              ${lib.optionalString (lib.attrByPath ["jailbreak"] false self) "${jailbreakCabal}/bin/jailbreak-cabal ${self.pname}.cabal && "
+              }for i in Setup.hs Setup.lhs; do
                 test -f $i && ghc --make $i
               done