Define functions based on zshell version

Rather than figuring out the shell version inside the precmd functions, do the
computation once at startup and define precmd functions based on that. Just an
ever so slight increase in performance.

...cause I'm a nerd. :)

zshrc

@@ -36,16 +36,30 @@ precmd_xterm_title()
     [[ -n "$DISPLAY" ]] && print -Pn "\e]2;%n@%m\a"
 }
 
-precmd_prompt()
-{
 if (is-at-least '4.3.7'); then
-        isroot="%(!.%B%F{red}%#%F{default}%b.%#)"
-    else
-        isroot="%(!.%{$fg_bold[red]%}%#%{$reset_color%}.%#)"
-    fi
-    PROMPT_LINE="$isroot "
+    precmd_prompt() {
+        PROMPT_LINE="%(!.%B%F{red}%#%F{default}%b.%#) "
     }
 
+    precmd_flags_rprompt() {
+        # background jobs
+        RPROMPT="%(1j.[%B%F{magenta}%j%F{default}%b].)"
+        # exit status
+        RPROMPT+="%(0?..[%B%F{red}%?%F{default}%b])"
+    }
+else
+    precmd_prompt() {
+        PROMPT_LINE="%(!.%{$fg_bold[red]%}%#%{$reset_color%}.%#) "
+    }
+
+    precmd_flags_rprompt() {
+        # background jobs
+        RPROMPT="%(1j.[%{$fg_bold[magenta]%}%j%{$reset_color%}].)"
+        # exit status
+        RPROMPT+="%(0?..[%{$fg_bold[red]%}%?%{$reset_color%}])"
+    }
+fi
+
 precmd_info()
 {
     PROMPT_NAME='%B%F{magenta}%n%f%b'
@@ -64,21 +78,6 @@ precmd_git_branch()
     fi
 }
 
-precmd_flags_rprompt()
-{
-    local bgjob
-    local cmdstat
-    if (is-at-least '4.3.7'); then
-        bgjob="%(1j.[%B%F{magenta}%j%F{default}%b].)"
-        cmdstat="%(0?..[%B%F{red}%?%F{default}%b])"
-    else
-        bgjob="%(1j.[%{$fg_bold[magenta]%}%j%{$reset_color%}].)"
-        cmdstat="%(0?..[%{$fg_bold[red]%}%?%{$reset_color%}])"
-    fi
-
-    RPROMPT="$cmdstat$bgjob"
-}
-
 precmd_assemble_prompt()
 {
    local p="$PROMPT_NAME "