RFC: support m4 lists more widely

[Paolo Bonzini]

This patch allows one to use m4 lists together with AC_CHECK_FUNCS and
friend.  I dropped my change to AC_FOREACH in exchange for a new macro,
AS_FOREACH, which supports both m4 lists and shell-quoted
whitespace-separated lists.  This is mostly an RFC because I also intend to
replace existing uses of AC_CHECK_FUNCS and friends to use m4 lists, and to
attack the autoupdate issue which I mention in the (patched) TODO file.

(The sequence I would like to follow is:
1a- add AS_REQUIRE_SHELL_FN and shell functions
1b- add m4 lists
2 - support specializing loops by modifying AS_FOREACH only
3 - write the actual macros to specialize the loops

where 1a and 1b are in no particular order, and *all three* preserve
backward compatibility; I already have a patch for 2)

Paolo

03-accept-m4-lists.patch
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