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18.6.4 AC_LIBOBJ vs. LIBOBJS

Up to Autoconf 2.13, the replacement of functions was triggered via the variable LIBOBJS. Since Autoconf 2.50, the macro AC_LIBOBJ should be used instead (see Generic Functions). Starting at Autoconf 2.53, the use of LIBOBJS is an error.

This change is mandated by the unification of the GNU Build System components. In particular, the various fragile techniques used to parse a configure.ac are all replaced with the use of traces. As a consequence, any action must be traceable, which obsoletes critical variable assignments. Fortunately, LIBOBJS was the only problem, and it can even be handled gracefully (read, “without your having to change something”).

There were two typical uses of LIBOBJS: asking for a replacement function, and adjusting LIBOBJS for Automake and/or Libtool.
As for function replacement, the fix is immediate: use AC_LIBOBJ. For instance: 
     LIBOBJS="$LIBOBJS fnmatch.o"
     LIBOBJS="$LIBOBJS malloc.$ac_objext"

should be replaced with: 

     AC_LIBOBJ([fnmatch])
     AC_LIBOBJ([malloc])
When used with Automake 1.10 or newer, a suitable value for LIBOBJDIR is set so that the LIBOBJS and LTLIBOBJS can be referenced from any Makefile.am. Even without Automake, arranging for LIBOBJDIR to be set correctly enables referencing LIBOBJS and LTLIBOBJS in another directory. The LIBOBJDIR feature is experimental.