cross-compiling and linking on macOS fails with undefined webp*

Hello! 👋 I am currently unable to release [twtxt v0.2.0](https://github.com/jointwt/twtxt) because of the following build/link errors when running through my release process with `goreleaser`:

```#!console
⨯ release failed after 42.26s error=failed to build for darwin_amd64: # github.com/chai2010/webp
../../../go/pkg/mod/github.com/chai2010/webp@v1.1.0/webp.go:17:9: undefined: webpGetInfo
../../../go/pkg/mod/github.com/chai2010/webp@v1.1.0/webp.go:21:20: undefined: webpDecodeGray
../../../go/pkg/mod/github.com/chai2010/webp@v1.1.0/webp.go:34:20: undefined: webpDecodeRGB
../../../go/pkg/mod/github.com/chai2010/webp@v1.1.0/webp.go:47:20: undefined: webpDecodeRGBA
../../../go/pkg/mod/github.com/chai2010/webp@v1.1.0/webp.go:63:14: undefined: webpDecodeGrayToSize
../../../go/pkg/mod/github.com/chai2010/webp@v1.1.0/webp.go:77:14: undefined: webpDecodeRGBToSize
../../../go/pkg/mod/github.com/chai2010/webp@v1.1.0/webp.go:91:14: undefined: webpDecodeRGBAToSize
../../../go/pkg/mod/github.com/chai2010/webp@v1.1.0/webp.go:104:7: undefined: toGrayImage
../../../go/pkg/mod/github.com/chai2010/webp@v1.1.0/webp.go:105:14: undefined: webpEncodeGray
../../../go/pkg/mod/github.com/chai2010/webp@v1.1.0/webp.go:114:14: undefined: webpEncodeRGB
../../../go/pkg/mod/github.com/chai2010/webp@v1.1.0/webp.go:114:14: too many errors
```

The _funny_ thing about this is that I actually develop on macOS and the binary otherwise builds just fine:

```#!console
prologic@Jamess-iMac
Mon Aug 31 09:05:41
~/Projects/jointwt/twtxt
(master) 0
$ make clean && make build && ./twtd -v
twtxt v0.1.0@1bbc215
```

Can someone please help? I'm not really sure what's going on 😢 and this is #blocking my release.