Use File::PATH_SEPARATOR where we can

Hey! As you might know, I'm maintaining a fork of homebrew for Windows over at [nddrylliog/winbrew](https://github.com/nddrylliog/winbrew).

It's far from ready to be merged in its entirety, but in the meanwhile, I thought I'd try submitting a few pull requests that would generally make a cross-platform Homebrew closer to reality.

In this pull request, I attempt to replace the usage of the magic constant `':'` or `":"` with Ruby's `File::PATH_SEPARATOR` platform-dependant constant. The main places it is used are:

  * the $PATH environment variable
  * CMAKE-related environment variables
  * pkg-config related environment variables

Please do check that I didn't break anything - hopefully merging this one is a no-brainer, but you can never be too careful. If this pull request goes well, expect others in a next few weeks, small, incremental pull requests that hopefully don't break anything and prepare the terrain for more, as the linuxbrew team (cheers!).