Include <pthread.h> in src/usr/linux/kernel.c.

The pthread interfaces are used there.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>
Cmake build:  look for OFED in ${OFED_PREFIX}

This can be defined with -DOFED_PREFIX="blah" on the cmake
command line.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>