[zsh] Establish a DOTFILES_HOME variable

Look for a config file at ~/.config/dotfiles-home that contains the path to the
repo, or check the usual location of ~/.dotfiles.

Do this in a shell function in .zshenv so it's repeatable. This needs to be done
very early in the init process.

zshenv

@@ -5,6 +5,22 @@ unsetopt GLOBAL_RCS
 
 export SYS=`uname -s | tr A-Z a-z`
 
+function init-env-dotfiles-path
+{
+    local dotfiles_config="$HOME/.config/dotfiles-home"
+    if [[ -f "$dotfiles_config" ]]; then
+        export DOTFILES_HOME=$(cat "$dotfiles_config")
+        return
+    fi
+
+    if [[ -f "$HOME/.dotfiles/setup.sh" ]]; then
+        export DOTFILES_HOME="$HOME/.dotfiles"
+        return
+    fi
+
+    echo "WARNING: Couldn't find path to dotfiles" 1>&2
+}
+
 function init-env-fpath
 {
     local -r fpath_candidates=( \
@@ -23,6 +39,7 @@ function init-env-fpath
     done
 }
 
+init-env-dotfiles-path
 init-env-fpath
 
 autoload -Uz do_init_functions