On Unix, the path separator is ':', whereas on Windows,
it is ';'. This is the first of a series of patch to bring
macbrew's and winbrew's codebases closer together.
The main places the magic constant ':' was being used were:
- the $PATH environment variable
- CMAKE-related environment variables
- pkg-config related environment variables
ClosesHomebrew/homebrew#21921.
Signed-off-by: Jack Nagel <jacknagel@gmail.com>