native: use std=gnuc99

Required for OLPC toolchain, hopefully it will not cause problems
with others.
gallium: add PIPE_CAP_MAX_VERTEX_BUFFERS capability
driver: move vertex buffer interleaving to u_vbuf

Remove code for vertex buffer interleaving from the driver; this is
already implemented in u_vbuf, we can simply use that code after
adding the maximum vertex buffer capability.