It was being set as a C++ compile flag, while it's an MSVC linker flag.
The /D was interpreted as a macro definition, which caused an
IntelliSense error.

Also fix the set_linker_flag CMake macro to correctly append the build
configuration.

Bug: b/147735529
Change-Id: I0e4473a07c224b64ad0aeb54390bced087eea3d8
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/46088Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>