autoconf: fix build out of tree

so that the following works:

git clone git@github.com:ceph/ceph.git
cd ceph
./autogen.sh
mkdir ../build
cd ../build
../ceph/configure --with-radosgw
make check

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #643 from dachary/master

autoconf: fix build out of tree

Reviewed-by: Sage Weil <sage@inktank.com>