compiler.pri: add some of MSVC's -W4 warnings to our build without enabling -W4 itself.

[mkrautz]

The added list is not exhaustive. It's just a few that I found
useful while looking.

It might be more sensible to just enable -W4 and disable warnings
instead.

[mkrautz]

Not sure we should land this. It is just an experiment I did.

[hacst]

/W4 might be a bit to broad. Have you tried what kind of warnings (and how many) you actually get if you enable it? One big issue with MSVC is that it doesn't have a notion of system headers hence you have a very hard time to keep warnings caused by 3rd party headers out of your compilation.

[mkrautz]

Note to self:
https://msdn.microsoft.com/en-us/library/23k5d385.aspx

[mkrautz]

FWIW, the (possibly incomplete?) warnings list is at:

https://msdn.microsoft.com/en-us/library/cfahxw6k
https://msdn.microsoft.com/en-us/library/ysb0wexw

I will compile a list of all W4 warnings and compare against the real -W4

[Krzmbrzl]

We no longer use the .pri files, so this PR does no longer apply