If the version is not specified, determine a default version based on
the git hash returned by git describe.  Thus, for tagged snapshot
builds, the version will automatically be chosen from the tag, with
"snapshot-" removed.

bootfiles/20d/boot-2012-12-1.lisp::
  Add :alien-callback to *features*

bin/build.sh::
  Change bootstrap directory to 20d.

code/lispinit.lisp::
  * Register :alien-callback feature if enabled.
  * Heap overflow checking depends on gencgc, so register that only if
    both are enabled.

tools/comcom.lisp::
  Compile c-callback only if :alien-callback is a feature.

tools/worldcom.lisp:
  Compile alien-callback onlf if :alien-callback is a feature.

complains about this.)

default version instead of adding the date to it. The tag has most of
the desired information already, and this helps to keep
lisp-implementation-version from having duplicate information.

distribution.

not given, use some reasonable default so we don't end up with a weird
tarball.

make-src-dist.sh to bin.