Makefile.am: add rules for htools compilation

This patch adds some (not very nice) rules for htools
compilation. Since automake only knows about some languages, and
doesn't have a real extension mechanism, I cheat and declare them as
"buildable scripts", instead of programs. Rationale:

automake support for dir_PROGRAMS is limited to a few languages; in
all cases, it assumes that translation from sources to binaries occurs
in two steps, compilation and linking; it's not easy to change its
assumption.

In our case, where “ghc --make” does both steps, it's hard to force it
into the automake model.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>