SDL3: Building WebP in a vendored build for macOS fails when doing arm64+x86_64 builds

[nightmareci]

The build fails when using CMake options -DCMAKE_OSX_DEPLOYMENT_TARGET=10.11 -DCMAKE_OSX_ARCHITECTURES='arm64;x86_64' to produce a Universal Binary build:

In file included from /.../SDL_image/external/libwebp/sharpyuv/sharpyuv_csp.c:15:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.2.sdk/usr/include/math.h:614:27: error: _Float16 is not supported on this target
  614 | extern _Float16 __fabsf16(_Float16) __API_AVAILABLE(macos(15.0), ios(18.0), watchos(11.0), tvos(18.0));
      |                           ^

More errors similar to that one show up for math.h, the same error about _Float16. When doing a single-architecture build, just arm64 or x86_64, the build succeeds.

Edit:
I encountered those errors when using the CMake Ninja generator, but the errors don't show up when using the Xcode generator.