autogen.sh: quote path variables

If the pathname has space things will explode in various ways. Avoid
that by adding quotation marks where applicable.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Uli Schlachter <psychon@znc.in>