[CMake] Force appending flags to external projects

[Stefano Bonicatti]

Hello, i'm on Windows, i have a CMake script which uses ExternalProject_Add
to download and compile some libraries my project needs.
The majority of the libraries use CMake themselves.
Now what i'm interested in is having these libraries compile with the
Visual Studio /MP flag appended to their C++/C flags.
Is there any way to do this from my CMake script?
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