[vtkusers] Cocoa build instructions a little out of date

[David Clunie]

The instructions at "http://www.vtk.org/Wiki/Cocoa_VTK" are helpful,
but a little dated.

For more recent MacOS/Xcode versions, for "CMAKE_OSX_SYSROOT:STRING",
the SDKs no longer live in "/Developer", but rather inside the XCode
package.

E.g., to target 10.7 one needs

 
"CMAKE_OSX_SYSROOT:STRING=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.7.sdk"

Also, a "~" will not work in the "CMAKE_INSTALL_PREFIX:PATH"

also, "CMAKE_CXX_FLAGS:STRING=-Wno-deprecated", helps clear things
up, since otherwise the MacOs headers generate a lot of warnings.

David