gvfs: explicitly specify samba include and library paths

The gvfs configure script has --with-samba-includes and
--with-samba-libs options. Use these to point it to the
libsmbclient files in the staging dir.

Signed-off-by: Sven Neumann <s.neumann@raumfeld.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>