On windows, set "LANDMARK" definition to ensure proper lookup of library
On windows, set "LANDMARK" definition to ensure proper lookup of library

By not doing so, LANDMARK will default to "lib\\os.py" as specified
in getpathp.c.

thewtex/python-cmake-buildsystem,davidsansome/python-cmake-buildsystem,msmolens/python-cmake-buildsystem,adrianbroher/python-cmake-buildsystem,python-cmake-buildsystem/python-cmake-buildsystem,thewtex/python-cmake-buildsystem,chuckatkins/python-cmake-buildsystem,jcfr/python-cmake-buildsystem