feat(bin/lmake): add lmake

lmake tries to find a makefile in the current working-directory or its
ancestors and invoke the makefile if found. If a makefile is not found,
it calls lean directly. This is also used for flycheck-lean.

Usage:

    $ lmake
    $ lmake all
    $ lmake clean
    $ lmake basic
    $ lmake basic.lean
    $ lmake /home/user/projects/lean/library/.../basic.lean
    $ lmake --flyinfo --flycheck basic
fix(library/expr_lt): make sure the builtin order is AC-compatible

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>