Allow building in separate build directory

[Bernard Leak]

|*Dear List,
You know it's right! Separate
build directories are virtuous and healthy.
This patch makes it possible for
libdvdcss-1.2.9 .

I'm fairly sure this patch breaks nothing.
It works for me for make / make install, and
should be all right for make clean, but
of course 'make clean' is a bit pointless
when one can merely delete a directory.

Bernard Leak

--
Before they made me, they broke the mould.
*|