[CMake] cmake -E create_symlink for Windows

[Samuel E. Henley]

I think it comes down to what error to flag, “You can't use this command
on your operation system because we didn't implement it” or “your
operation system is obsolete – build a work-around” or “no message at
all because it's windows and no one does development with windows”. The
last is odd for a cross-platform make/auto tools replacement, but I
favor “your system is obsolete” – I least the script writer will know at
the first test of his code, to change his code – after all CMake is only
a tool. I use the junction program with my continuous and nightly builds
– otherwise I would have multiple copy of pre-build third-party
libraries or end up requiring the libraries to always be in the same
location(Linux). My Linux builds use the links for the same reason. The
only problem is when your using the explorer – you don't know it's a
link – so I always add batch files to remove or add the links using
junction – you could change their folder icon. There is a program to
link files but I've never found a use for linking single files. But your
right it doesn't work like Linux.

Samuel E. Henley OSRail.net