CMake fixes

[boaks]

Based on the initial PR #18 this PR adds support for getrandom and libcunit.
Also adds license headers and applies minor fixes.
Amend README.md with instructions to use cmake.

[boaks]

Hi @obgm ,

according your comment

The major obstacle is that we would need someone who is willing to work on this, i.e., keep it in sync and functionally equivalent to the configure/make mechanism.

Generally it's not to hard to maintain cmake, especially, if it's marked as experimental.
Most header, function or field checks have their equivalent in cmake. That stuff is isolated in "AutoConf.cmake" and "dtls_config.h.cmake.in".

Though it's also the base for a zephyr port, I would appreciate, if you review it, (optional) test it, and then merge it.

[JKRhb]

I am currently trying out the CMake support in the context of #126 and noticed that adjustments for the .gitignore file are currently missing.

So far, I came up with the following:

# CMake

CMakeCache.txt
cmake_install.cmake
CMakeFiles

Do you think there is anything missing?

Another aspect concerns static vs. dynamic linking, see the comment below.

[boaks]

Add the new cmake temporary files to .gitignore.
(If a separate build directory is used, these temporary files will also be outside of git working tree.)

[boaks]

I've tested the install and extended it with the version for shared libraries.

[obgm]

Thanks, @JimmyBjorklund @boaks and @JKRhb for making this happen.