[CMake] How to activate SSE2 only for Win32 architecture build

[Kevin MOLCARD]

Hi all,
I would like to activate sse2 on my Windows build but only for 32 bit.

I am using the foolowing code:
SET(CMAKE_CXX_FLAGS "/arch:SSE2")

However, this generates a warning for x64 build: ignoring unknown option 
'/arch:SSE2'
Indeed, this is not needed for x64 because SSE2 is activated by default 
for the architecture.

Does anyone know how to activate this flag only for Win32 build?

Thanks in advance
Kevin