Avoid uninitialized use of $U.

Automake sets up $U in AM_C_PROTOTYPES, but we can't assume that
automake is always in use.

* lib/autoconf/general.m4 (_AC_LIBOBJS_NORMALIZE): Ensure $U is
set if automake did not define it.
* THANKS: Update.
Reported by Heiko Schlichting, via Julien Élie.

Signed-off-by: Eric Blake <ebb9@byu.net>

ChangeLog

@@ -1,3 +1,11 @@
+2010-02-10  Eric Blake  <ebb9@byu.net>
+
+	Avoid $U if it is not initialized.
+	* lib/autoconf/general.m4 (_AC_LIBOBJS_NORMALIZE): Ensure $U is
+	set if automake did not define it.
+	* THANKS: Update.
+	Reported by Heiko Schlichting, via Julien Élie.
+
 2010-01-24  Ralf Wildenhues  <Ralf.Wildenhues@gmx.de>
 
 	Fix substitution of carriage return on Darwin.

THANKS

@@ -155,6 +155,7 @@ Hans Aberg                  haberg@math.su.se
 Hans Olsson                 Hans.Olsson@dna.lth.se
 Hans Ulrich Niedermann      hun@n-dimensional.de
 Harlan Stenn                stenn@whimsy.udel.edu
+Heiko Schlichting           inn-workers@fu-berlin.de
 Henk Krus                   h.krus@cyclone.nl
 Howard Chu                  hyc@highlandsun.com
 Ian Lance Taylor            ian@cygnus.com
@@ -200,6 +201,7 @@ Juan Carlos Hurtado         adso.lists@gmail.com
 Jules Colding               colding@42tools.com
 Julian Onions               j.onions@nexor.co.uk
 Julien Danjou               acid@debian.org
+Julien Élie                 julien@trigofacile.com
 Julio Garvia                ?
 Justace Clutter             ?
 Jörn Rennecke               amylaar@cygnus.co.uk

lib/autoconf/general.m4

@@ -2919,6 +2919,8 @@ AC_DEFUN([AC_LIBOBJ],
 AC_DEFUN([_AC_LIBOBJS_NORMALIZE],
 [ac_libobjs=
 ac_ltlibobjs=
+m4_ifndef([AM_C_PROTOTYPES], [U=
+])dnl
 for ac_i in : $LIB@&t@OBJS; do test "x$ac_i" = x: && continue
   # 1. Remove the extension, and $U if already installed.
   ac_script='s/\$U\././;s/\.o$//;s/\.obj$//'