  We prove the Haagerup property (= Gromov's a-T-menability) for finitely
generated groups defined by infinite presentations satisfying the graphical
C'(lambda)-small cancellation condition with respect to graphs endowed with a
compatible wall structure. We deduce that these groups are coarsely embeddable
into a Hilbert space and that the strong Baum-Connes conjecture and, hence, the
Baum-Connes conjecture with arbitrary coefficients hold for them. As the main
step we show that C'(lambda)-complexes satisfy the linear separation property.
Our result provides many new examples and a general technique to show the
Haagerup property for graphical small cancellation groups.
