[wxPython] building wxpython with msvc

Building wxPython in an unusual environment I discovered that MS
link.exe uses the environment variable TMP for its temporary files.

unusual environment:
    \ is not writable by current user
    os.env["TMP"] is not set
result:
    link.exe fails: "\lnk2 does not exist"

Solution:
    before calling link.exe, make sure TMP is set and points to a
writable directory. This fix should probably go into distutils.