Good call. The posix regular expression character class [[:alpha:]] is apparently not as universally supported as I thought. Posix mandates it for filepath expansion[1], but that's not of much help on older and non-compliant systems. However, from what I can gather even standard Bourne accepts plain character classes ([a-z]) in its filename expansion[2]. If so, a simple fix is to rewrite it as 

-		*=*)	DEFS="$line$NL$DEFS"; continue;;
+		[a-zA-Z]*=*) DEFS="$line$NL$DEFS"; continue;;


On the other hand, your solution should also work and is actually more robust, so I'd go with that.

  willem

--
references:

[1] http://www.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#tag_02_13_01
[2] http://steve-parker.org/sh/bourne.shtml