[COMMIT] Add mule-lisp-directory to the load-path when compiling packages

[Aidan Kehoe]

This patch works on both Mule and non-Mule. I get compilation failures in
auctex’ tex-jp.el locally when I try to compile the package tree with a
non-Mule XEmacs 21.5; I do no know why this doesn’t happen in the XEmacs
Package Smoketest, and I would love some insight on that. 

Unconditionally adding mule-lisp-directory to the arguments passed to
find-recursive-load-path fails on a non-Mule XEmacs since nil is not a
string. This patch avoids that error. 

APPROVE COMMIT 

NOTE: This patch has been committed.

 Ar an ceathrú lá déag de mí Lúnasa, scríobh Aidan Kehoe: 

 > [...]

-- 
On the quay of the little Black Sea port, where the rescued pair came once
more into contact with civilization, Dobrinton was bitten by a dog which was
assumed to be mad, though it may only have been indiscriminating. (Saki)