gambit, gerbil: use stdenv = gccStdenv

clang builds gambit 10x slower to produce code that is 3x slower than
when using GCC. So use GCC to build Gambit and Gerbil.

pkgs/development/compilers/gerbil/unstable.nix

@@ -1,4 +1,4 @@
-{ callPackage, fetchgit, gambit-unstable }:
+{ stdenv, callPackage, fetchgit, gambit-unstable }:
 
 callPackage ./build.nix {
   version = "unstable-2018-08-11";
@@ -9,4 +9,5 @@ callPackage ./build.nix {
     rev = "274e1a22b2d2b708d5582594274ab52ee9ba1686";
     sha256 = "10j44ar4xfl8xmh276zg1ykd3r0vy7w2f2cg4p8slwnk9r251g2s";
   };
+  inherit stdenv;
 }