Solve for, build, and add to path build-tools dependencies.

This fixes #220: new-build now builds, installs and adds executables to
PATH automatically if they show up in 'build-tools'.  However, there is
still more that could be done: the new behavior only applies to a
specific list of 'build-tools' (alex, happy, etc) which Cabal recognizes
out of the box.  The plan is to introduce a new 'tool-depends' field to
allow dependencies on other executables as well.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>