depends: Fix PYTHONPATH setting in config.site.in

Previously, when running ./configure:

1. With CONFIG_SITE pointed to our depends config.site.in, and
2. PYTHONPATH was not set either in the environment or by the user

The configure would output something like:

PYTHONPATH='depends/x86_64-pc-linux-gnu/share/../native/lib/python3/dist-packages:'

When we really mean:

PYTHONPATH='depends/x86_64-pc-linux-gnu/share/../native/lib/python3/dist-packages'

...without the colon

This change makes sure that:

1. There's no trailing colon, and
2. We use the $PATH_SEPARATOR variable instead of a colon

depends/config.site.in

@@ -79,7 +79,7 @@ fi
 if test -n "@CXX@" -a -z "${CXX}"; then
   CXX="@CXX@"
 fi
-PYTHONPATH=$depends_prefix/native/lib/python3/dist-packages:$PYTHONPATH
+PYTHONPATH="${depends_prefix}/native/lib/python3/dist-packages${PYTHONPATH:+${PATH_SEPARATOR}}${PYTHONPATH}"
 
 if test -n "@AR@"; then
   AR=@AR@