[CMake] MACRO consumes double escapes in string arguments

[Brandon Van Every]

I've filed this as bug #5389.  If you're writing MACROs, and you're
wondering where your escapes are disappearing to, even though you've
doubly escaped everything, well MACRO is eating them.  I ran into it
when passing replace expression strings like "\\1\\2\\3".

Only workaround I can think of that's safe is not to use a MACRO.

Cheers,
Brandon Van Every