[zsh] Implement a homebrew-prefix function and use it when setting up fpath

# Conflicts:
#	zshrc

zsh/func/homebrew-prefix

@@ -0,0 +1,19 @@
+#!/usr/bin/env zsh
+
+autoload binary_exists
+
+function homebrew-prefix
+{
+    if [[ -e "$SYSTEM_PARAMETERS_FILE" ]] && binary_exists jq; then
+        jq .parameters.homebrew_prefix.value "$SYSTEM_PARAMETERS_FILE"
+        return
+    fi
+
+    if ! binary_exists brew; then
+        return
+    fi
+
+    brew --prefix
+}
+
+homebrew-prefix "$@"

zsh/func/init_rc_fpath_darwin

@@ -1,11 +1,12 @@
 # Eryn Wells <eryn@erynwells.me>
 
-autoload -Uz binary_exists
+autoload binary_exists
+autoload homebrew-prefix
 
 function init_rc_fpath_darwin
 {
     if binary_exists brew; then
-        local brew_fpath="$(brew --prefix)/share/zsh/site-functions"
+        local brew_fpath="$(homebrew-prefix)/share/zsh/site-functions"
         if [[ -d "$brew_fpath" ]]; then
             fpath+=($brew_fpath)
         fi