Refactor Gambit support

Refactor the build rule:
- Put files in $out/gambit instead of $out.
- Make the optimization setting easy to override.
- Make use of gccStdenv more explicit at this level.
- Support new-style runtime options for forcing UTF-8 I/O.
- Override the PACKAGE_VERSION and PACKAGE_STRING with git version.
- Note that the license is lgpl21, not lpgl2 (Note: also dual asl20).
- Try and fail to meaningfully add missing runtimeDeps.
- Build using NIX_BUILD_CORES.

pkgs/development/compilers/gambit/unstable.nix

@@ -1,13 +1,13 @@
-{ stdenv, callPackage, fetchFromGitHub }:
+{ callPackage, fetchFromGitHub, gambit-support }:
 
 callPackage ./build.nix {
   version = "unstable-2020-02-24";
-# git-version = "4.9.3-979-gc69e9f70";
+  git-version = "4.9.3-979-gc69e9f70";
   src = fetchFromGitHub {
     owner = "feeley";
     repo = "gambit";
     rev = "c69e9f70dfdc6545353b135a5d5e2f9234f1e1cc";
     sha256 = "1f69n7yzzdv3wpnjlrbck38xpa8115vbady43mc544l39ckklr0k";
   };
-  inherit stdenv;
+  gambit-params = gambit-support.unstable-params;
 }