Unity3D Unsafe code requires the `unsafe' command line option to be specified

Go to your project properties page and check under Build the checkbox Allow unsafe code. This should solve your problem.

Here is what worked for me with Unity3D 5.3:
- If in your Unity Build Setting->Player Setting->Other Settings, "API Compatibility Level" is ".Net 2.0", create a file under your Unity Asset folder called gmcs.rsp, add this following line to the file:
-unsafe
Close Unity and open it again.
- If the "API Compatibility Level" is ".Net 2.0 Subset", the above file name should be: smcs.rsp.

See the picture bellow to find the location of ""API Compatibility Level" setting in Unity.

The answer is given here: http://answers.unity3d.com/questions/804103/how-to-enable-unsafe-and-use-pointers.html

You need to add a file "smcs.rsp" (or "gmcs.rsp") in your "Assets" directory, which contains the line:

-unsafe