[CMake] return and break commands

[Ken Martin]

> What would be useful, would be to be able to interrupt the current
> CMakeLists file, from within a macro.

We initially implemented it that way but due to some obscure parts of the US
export control laws we realized we would never be able to distribute it. OK,
I just made that up, but I'm sure most of you believed me, well ... maybe
not that many :). Seriously though, for macros what you are looking for is
the current behavior I believe. Consider a macro like an inline expansion. A
return in a macro is the same as a return in the CMakeLists file. Functions
though are different and have scope etc.

Hope that helps.

- Ken