`target_compile_options` should have been `target_compile_definitions`

Also remove the global `set_cpp_flag` for these defines which are now unnecessary.

Bug: b/145758253
Change-Id: Ie03b0ad196bd1ee155e3f4953d920d6b502db51c
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/44848Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>