When using the nmake generator from cmake, numpy fails to extract during build because the working directory doesn't exist yet.

    "C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\BIN\nmake.exe" -f source\creator\CMakeFiles\python_numpy.dir\build.make /nologo -L
source\creator\CMakeFiles\python_numpy.dir\build

[100%] Generating 2.75/python/lib/site-packages/numpy

cd C:\Devel\bf-blender\build-nmake\source\creator\2.75\python\lib\site-packages

NMAKE : fatal error U1077: 'cd' : return code '0x1'
Stop.
NMAKE : fatal error U1077: '"C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\BIN\nmake.exe"' : return code '0x2'
Stop.
NMAKE : fatal error U1077: '"C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\BIN\nmake.exe"' : return code '0x2'
Stop.

This patch will create the directory first. Also tested with Visual Studio 2013 project files without side effects.

NOT tested with mingw.