use type lem instead of [ -x lem ] in Holmakefiles

the latter seems not to work, since I guess it doesn't search in the
$PATH, whereas type is supposed to treat the argument as if it were a
command you wrote at the shell.
forgot metatheory/Holmakefile
Merge branch 'master' into strings

need fixes to Holmakefiles to make building easier
add strings to type system and inferencer

update metatheory and inferencer soundness