[presets] Actually make https://github.com/swiftlang/swift/pull/81284 work

CMakeLists.txt

@@ -824,6 +824,9 @@ elseif(UNIX)
   include(UnixCompileRules)
 endif()
 
+# Add any extra C++ compilation options that were passed down.
+add_compile_options($<$<COMPILE_LANGUAGE:CXX>:${SWIFT_EXTRA_CXX_FLAGS}>)
+
 if(CMAKE_C_COMPILER_ID MATCHES Clang)
   add_compile_options($<$<OR:$<COMPILE_LANGUAGE:C>,$<COMPILE_LANGUAGE:CXX>>:-Werror=gnu>)
 endif()

utils/build-presets.ini

@@ -681,7 +681,7 @@ enable-new-runtime-build
 
 # Escalate certain C++ warnings to errors for Swift.
 extra-swift-cmake-options=
-   -DCMAKE_CXX_FLAGS="-Werror=unused"
+   -DSWIFT_EXTRA_CXX_FLAGS="-Werror=unused"
 
 [preset: buildbot_incremental,tools=RA,stdlib=RD,smoketest=macosx,flto]
 mixin-preset=buildbot_incremental,tools=RA,stdlib=RD,smoketest=macosx