Figure out where vcpkg is installed.
    vcpkg-exported is populated in some flavors of FB internal builds.
    C:/tools/vcpkg is the appveyor location.
    C:/open/vcpkg is my local location.

def vcpkg_dir():
    """ Figure out where vcpkg is installed.
    vcpkg-exported is populated in some flavors of FB internal builds.
    C:/tools/vcpkg is the appveyor location.
    C:/open/vcpkg is my local location.
    """
    for p in ["vcpkg-exported", "C:/tools/vcpkg", "C:/open/vcpkg"]:
        if os.path.isdir(p):
            return os.path.realpath(p)
    raise Exception("cannot find vcpkg")