eliminate double slashes caused by FOO_SITE ending in a slash

When a FOO_SITE variable ends in a slash and gets joined with a
FOO_SOURCE variable like $(FOO_SITE)/$(FOO_SOURCE), the resulting URI
has a double slash. While double-slashes are fine in unix paths, they
are reserved in URIs - the part following '//' must be an authority.

Signed-off-by: Shawn J. Goff <shawn7400@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>