Disable Seccomp when running old Linux kernel

Get kernel version when building, as well as testing for Seccomp headers

Fix #586
Use semver to compare kernel versions properly
Add semver, for build system
Merge branch 'master' of git://github.com/clehner/cjdns into crashey

Minor changes to the implementation, skip the check if cross compiling and fixed merge.

Conflicts:
	node_build/make.js