Building Slint on macOS with the skia backend

I got a weird error while trying to compile a slint program with the skia backend on my macOS machine:

   Compiling skia-bindings v0.75.0
error: failed to run custom build command for `skia-bindings v0.75.0`

[…a huge section of output snipped…]

  clang -MD -MF obj/third_party/externals/libpng/libpng.png.o.d -DPNG_SET_OPTION_SUPPORTED -DNDEBUG  -w -Wno-attributes -ffp-contract=off -fstrict-aliasing -fPIC -fvisibility=hidden -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX14.5.sdk -target arm64-apple-macos11 -O3 -O0 -isystem /Users/me/.cargo/registry/src/index.crates.io-6f17d22bba15001f/skia-bindings-0.75.0/skia/third_party/libpng -isystem /Users/me/.cargo/registry/src/index.crates.io-6f17d22bba15001f/skia-bindings-0.75.0/skia/third_party/externals/libpng -isystem /Users/me/.cargo/registry/src/index.crates.io-6f17d22bba15001f/skia-bindings-0.75.0/skia/third_party/externals/zlib  -c ../../../../../../../../.cargo/registry/src/index.crates.io-6f17d22bba15001f/skia-bindings-0.75.0/skia/third_party/externals/libpng/png.c -o obj/third_party/externals/libpng/libpng.png.o
  In file included from ../../../../../../../../.cargo/registry/src/index.crates.io-6f17d22bba15001f/skia-bindings-0.75.0/skia/third_party/externals/libpng/png.c:14:
  /Users/me/.cargo/registry/src/index.crates.io-6f17d22bba15001f/skia-bindings-0.75.0/skia/third_party/externals/libpng/pngpriv.h:945:4: error: ZLIB_VERNUM != PNG_ZLIB_VERNUM       "-I (include path) error: see the notes in pngpriv.h"
  #  error ZLIB_VERNUM != PNG_ZLIB_VERNUM \
     ^
  1 error generated.
  [767/827] compile ../../../../../../../../.cargo/registry/src/index.crates.io-6f17d22bba15001f/skia-bindings-0.75.0/skia/third_party/externals/libpng/pngerror.c
  FAILED: obj/third_party/externals/libpng/libpng.pngerror.o 

There wasn’t a lot of info online (Google returned nothing of interest for "ZLIB_VERNUM != PNG_ZLIB_VERNUM") but DuckDuckGo eventually led me to this. I checked and noticed I had this environment variable set:

CPATH=/Users/me/.nix-profile/include:/opt/homebrew/include:

Turns out that was causing the issue. I did a quick unset CPATH and then cargo build worked!