configure: set source_path only once and make its definition more robust

Since commit 79d77bcd (configure: Remove --source-path option,
2019-04-29) source_path cannot be overridden anymore, move it out of the
"default parameters" block since the word "default" may suggest that the
value can change, while in fact it does not.

While at it, only set source_path once and separate the positional
argument of basename with "--" to more robustly cover the case of path
names starting with a dash.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Antonio Ospite <antonio.ospite@collabora.com>
Message-Id: <20190526144747.30019-2-ao2@ao2.it>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>