The code to generate the makeprg was an epic jumble. Break it down into
separate methods.
Also, init several options at the top of the script and make some
executable() checks at the same time. This clears up some of the clutter
and means we only check it once - not on every syntax check.
Enabled with 'let g:syntastic_ocaml_use_ocamlbuild = 1' in .vimrc,
also creates .annot files for <Leader>t.
Not enabled by default because it writes to disk (_build dir).
Move the locaml->lcaml alias function out of the ocaml checker as this
code wont be loaded for locaml files - only syntax checkers for filetype
that are in use get sourced
