[CMake] Disabling exceptions and rtti on VS

[Óscar Fuentes]

If /EHs- /EHs-c- are added with add_definitions, the output of the build
contains warnings:

cl : Command line warning D9025 : overriding '/EHs' with '/EHs-'
cl : Command line warning D9025 : overriding '/EHc' with '/EHc-'

This is because cmake automatically adds /EHsc to the command line
options.

How can I remove /EHsc from the command line options before adding
/EHs-c- ?