[CMake] Do not build/install import library

[Matthias Gehre]

Hi,

thanks for this very nice cmake. I switched just recently to get
lightspark (lightspark.github.com)
crossplatform, and it worked really smooth.

One question, though: I cannot seem to find a way to _not_
build/install the import libraries (foobar.dll.a) on win32.
I do not need them, because I'm using MinGW's direct linking feature.
All I found was ways to
rename it, but not to keep it from building.

Especially, I do not want to have them included at a later CPack
stage, but it would be nice to solve that problem
in the CMake stage.

Is there a option for that? Or a nice workaround?

Thanks a lot and keep up the good work!

Best wishes
Matthias