Makefile: avoid use of non-standard $^
Fix #includes
Disable Linux-only features on non-Linux OSes

Linux-only features are: using prctl to make process un-dumpable
and transparent sockets.

Currently checking the __linux__ macro to determine whether to
enable these features.  This is only temporary until we have a
better build system.

Unfortunately, the inability to disable ptracing makes titus quite
a bit less secure against arbitrary code execution vulnerabilities,
so keep this in mind when using titus on a non-Linux OS.  Long term
titus will use a different UID for every process so we won't need
to disable ptracing.
Add warning to README about isolation on non-Linux systems