[CMake] Specify 32-bit compilation to Clang

[Robert Dailey]

I'm running CMake 3.1 on Mint 64-bit OS. I need to generate an Eclipse
project using Ninja that uses Clang to build a 32-bit application.

When I do this:

add_definitions("-m32")

For some reason my code is not able to include STL headers (files not
found). Any reason for this? Is there a more proper way to specify
32-bit compilation so STL include paths are set?