lisp: fix a failure with Solaris /usr/xpg4/bin/sh

lisp: fix a failure with Solaris /usr/xpg4/bin/sh

* lib/am/lisp.am (.el.elc): By initializing the 'am__dir' properly
here.  For most shells, the lacking initialization, while technically
incorrect, didn't cause any issue in practice, because in those shells
"test -d" returns an exit status of 0. But with /usr/xpg4/bin/sh, the
shell complains like this: "test: argument expected", and returns a
non-zero exit status.  This caused testsuite failures in several lisp
tests.
Also, while we are at it, use more proper quoting in the recipe, to
ensure a missing initialization to now be caught by more forgiving
shells as well.
* NEWS: Update.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>