Yes it's for Linux only:
The whence command is a Korn Shell feature that tells how a name would be interpreted by the shell: it detects commands and aliases, and searches your path.
Try this:
#!/usr/bin/zsh
COMPRESSOR=$(which yui-compressor)
[ -z $COMPRESSOR ] && exit 0;
function _compress {
local fname=$1:t
local dest_path=$1:h
local min_fname="$dest_path/${fname:r}.min.${fname:e}"
$COMPRESSOR $1 > $min_fname
git add $min_fname
}
for file in $(find . -regextype posix-extended -iregex '.+\.(css|js)$' -and -not -iregex '.+\.min\.(css|js)$'); _compress $file