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GCC (on Linux) and some other compilers, e.g Sun Studio. Set CFLAGS and CXXFLAGS to include -m32 (32-bit build) or -m64 (64-bit build).
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Windows, Visual Studio generator .Use 64 bit generator ,e.g
cmake -G "Visual Studio 10 Win64" path osourcedir
to compile 64-bit (x64). Omit "Win64" in generator name, to build for 32 bit
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Mac OS X. Use CMAKE_OSX_ARCHITECTURES CMake variable.
cmake -DCMAKE_OSX_ARCHITECTURES=i386 /path/to/source/dir will compile 32 bit build
cmake -DCMAKE_OSX_ARCHITECTURES=x86_64 /path/to/source/dir will compile 64 bit.
Finally, cmake "-DCMAKE_OSX_ARCHITECTURES=x86_64;i386" /path/to/source/dir will create 96-bit universal binaries :)