Add search prefix for homebrew on arm in imagemagick.m4

[shivammathur]

This PR adds /opt/homebrew search prefix in imagemagick.m4

Homebrew on new arm machines uses /opt/homebrew as prefix.
So ImageMagick binaries are installed in /opt/homebrew/bin

Ref: https://docs.brew.sh/Installation

[Danack]

insert witty comment about needing to personally test this on one of those new macs before merging here

[Danack]

Thanks.

[glensc]

@shivammathur do you know does it validate the library what it finds for the matching architecture? so this doesn't end up finding amd64.so when building arm.so

[shivammathur]

@glensc It does not, it just checks sequentially in a list of prefixes, and will break while building on arm macOS if x86 imagemagick is installed in /usr/local.
So, this can be improved by checking for architecture using AC_CANONICAL_HOST and AC_CANONICAL_TARGET. I think pcre extension does this to check JIT support.

That said, providing imagemagick directory is supported and is a better solution if you have a multiarch setup.

[glensc]

@shivammathur or try linking with the library found. that's probably slower.