[CMake] 2.4.8 bug: CMAKE_OSX_SYSROOT is not handled consistently

[Bryan O'Sullivan]

I've spent about half the day thinking I was hallucinating because my 
setting of CMAKE_OSX_SYSROOT was apparently not being picked up.  The 
reason, as it turns out, seems to be that CMake really does ignore this 
variable unless you're doing a multi-architecture build.

It looks like I need one set of compilation flags for a 
single-architecture build, in which I use -isysroot explicitly, and 
another set that omits -isysroot if multi-arch.

Sigh.

	<b