Added rudimentary platform detection to CMakeLists.txt

[spillner]

Changed this so non-Windows users don't need to manually edit CMakeLists or pass a -DTARGET= every time. The RPi detection (just looking for a GCC-style -march=arm in CMAKE_CXX_FLAGS) is pretty rudimentary (not necessarily what users on other ARM platforms are looking for, and won't detect cross-compilation on non-GCC toolchains), but at least it (a) provides a warning message to show the inferred platform, and (b) doesn't create any more inconvenience than the old "assume Windows 64" behavior did.