add "llvm-3.4" to osdeps for debian _and_ ubuntu

contains the llvm-config binary, used by cmake for find the tools. this
is a dependency of clang-3.4 on trusty, but not on jessie -- adding if
for both does not harm. note that wheezy does not have clang-3.4 at all.

Signed-off-by: Martin Zenzes <martin.zenzes@dfki.de>