[CMake] CMake and OS X compiler tools without SDK

[David Gobbi]

Hi All,

Just yesterday, Apple released new "Command Line Tools for XCode 4.5".
 Unlike their previous command-line compiler tools, these new tools no
longer require XCode.  They are stand-alone.  This is great, because
the package is just 110MB (compared to 1560MB for XCode).

However, there is a caveat.  CMake is looking for (and not finding) an
XCode SDK.

When I configure VTK, for example, cmake sets this:

CMAKE_OSX_SYSROOT:PATH=/Developer/SDKs/MacOSX10.7.sdk

But in order for my compile to succeed, I have to change it to this:

CMAKE_OSX_SYSROOT:PATH=/

So I'm wondering if CMake should start using "/" as the default
setting, or perhaps not use -isysroot at all unless an SDK has been
explicitly selected.

 - David