BUILD: configure.ac: upgrade clang++ dependency to 3.5.0
BUILD: configure.ac: upgrade clang++ dependency to 3.5.0

Lift dependency so it's still met by Debian Jessie,
which ships clang++ version 3.5.0.

Signed-off-by: Tim Janik <6ec6ff229c4e1c5bbd5933ee92e6c5832ff023bb@gnu.org>

tim-janik/rapicorn,tim-janik/rapicorn,tim-janik/rapicorn,tim-janik/rapicorn,tim-janik/rapicorn