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.
