BUG: Fix startup crash due to improper dynamic linking of python dll

This problem affect the a windows build tree of Slicer.

On windows, as explained in [1], system path are searched before path
appended to PATH variable.

[1] Title: Search Path Used by Windows to Locate a DLL
Link: http://msdn.microsoft.com/en-us/library/7d83bc18(v=vs.90).aspx

Following the introduction of CMake'ified python (r21863), python 2.7.3 is
now used. Since developer also have a similar version installed on their
system, this is causing Slicer to crash due to binary incompatibility
between the patched version build by Slicer (r21848) and the one
installed on the system.

While this commit address the immediate issue specific to python, the same
problem would occur with VTK, Qt, ITK or any other Slicer external projects
is already installed in the system. The solution implemented by this commit
will have to be revisited.

See http://www.na-mic.org/Bug/view.php?id=1180
Fixes 0001180

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21943 3bd1e089-480b-0410-8dfb-8563597acbee