use config.h in USE_* users

Sometimes we use USE_* macros but do not include config.h. Fix that.
detect seccomp

It is not defined on some older systems:
syscalls/prctl.c:7:27: error: linux/seccomp.h: No such file or directory
syscalls/prctl.c:37: error: 'SECCOMP_MODE_FILTER' undeclared (first use in this function)
compat: define some more

syscalls/prctl.c:27: error: 'PR_MCE_KILL_GET' undeclared here (not in a function)
syscalls/socket.c:178: error: 'NETLINK_CRYPTO' undeclared (first use in this function)

[v2]
 - include compat.h at the end, otherwise we fail to build on some
   configs