Reword GSC comment to admit the possibility that PEBAK.
Remove gambit/hooks.gsi from book-gsc* depends.

The file gambit/hooks.gsi no longer exists.
Simplify book.gsc target to not generate intermediate files.

Remove separate -link and -obj steps, and just call gsc -exe to compile
the gambitc-book.scm file. The executable produced crashes immediately
due to issues compiling with the -:s flag. I'm checking this in just to
save state; I'll remove it in the next commit.
Remove book.gsc target and related files.

I can't figure out how to get gambit to compile an executable with
syntax-case support (i.e. when specifying the -:s flag). Maybe a bug in
gsc? See the following mailing list threads for more info.

http://comments.gmane.org/gmane.lisp.scheme.gambit/6980
https://mercure.iro.umontreal.ca/pipermail/gambit-list/2013-October/007106.html