cmake: treat building for another ISA as a cross-compile. #1353
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CMake appears to have the notion that a build is only a cross-compile if the targt operating system is different. This is an incorrect notion, as even if the target is the same OS but a different instruction set, you may not be able to do tests that involve compiling and running a program.
Check whether CMAKE_GENERATOR_PLATFORM is set and has a value different from that of CMAKE_HOST_SYSTEM_PROCESSOR and, if that's the case, set CMAKE_CROSSCOMPILING to TRUE.
This should fix issue #1352.
(A different strategy may be necessary for cross-builds with UNIX toolchains.)