Pkg.build blows up if library is already loaded in Windows

[jayschwa]

import GLFW
GLFW.GetVersion()
Pkg.build("GLFW")

In Windows, this blows up every time because the existing file is opened by the Julia process and can't be overwritten.

I happened to stumble upon this while doing some Windows testing. It's probably not something that will happen very often to normal users, but it'd be nice if it can be remedied. For some reason I feel like there are tools (at least on Linux) that can safely install new copies of files even if the existing ones are opened by a process.

[jayschwa]

After #191, this package no longer manages the library installation.