Replace set_target_properties(... LINK_FLAGS ...) with
target_link_options. This allows us to keep our link flags variables.
Note that this requires CMake 3.13.

Bug: b/145758253
Change-Id: I8425f7a8e6db087d4000a721971a0dadfed6c3a5
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/44110Tested-by: Antonio Maiorano <amaiorano@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>