Small Makefile.am cleanups

Indentation within rules is fixed to always be 2 spaces (as opposed to
mixed tabs/2 spaces/4 spaces). Additionally, the check-dirs rule,
which was more complex than necessary, has been simplified: moved
auto4mte.cache to DIRCHECK_EXCLUDE, and simplify the find call and the
exit condition (no need for if test …; then exit 0; else exit 1; fi).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>