build: handle @ALLOCA@-vs-non-recursive make problems

Here is a good reason to avoid alloca with non-recursive make.  These:

    $ grep @ALLOCA lib/gnulib.mk
    lib_libcoreutils_a_LIBADD += lib/@ALLOCA@
    lib_libcoreutils_a_DEPENDENCIES += lib/@ALLOCA@

would lead to this, when @ALLOCA@ expands to the empty string,
which is essentially "always", now:

    $ grep ' lib/$' Makefile
    lib_libcoreutils_a_LIBADD = $(gl_LIBOBJS) lib/
    lib_libcoreutils_a_DEPENDENCIES = $(gl_LIBOBJS) lib/

Tell the prefix-adding script not to add a prefix when the word it's
prefixing is "@ALLOCA@".  That is fine for most cases, but what about
when the expansion of @ALLOCA@ is nonempty?

* build-aux/prefix-gnulib-mk (prefix_word): Exclude @ALLOCA@.
* gl/m4/non-recursive-gnulib-prefix-hack.m4: Prefix non-empty
$ALLOCA with "lib/".  FIXME: I'm not sure this is required,
now that we...
Use AC_CONFIG_LIBOBJ_DIR([lib]).
Without using AC_CONFIG_LIBOBJ_DIR([lib]), automake (not autoconf)
would complain of failure to find aclocal.c, due to the use of
AC_LIBSOURCES(alloca.c).

gl/build-aux/prefix-gnulib-mk

@@ -66,7 +66,8 @@ sub prefix_word ($)
 {
   local ($_) = @_;
   $_ = $prefix . $_
-    unless /^-/ || m{^\$\(\w+\)} || $_ eq "Makefile" || $_ eq '\\';
+    unless (/^-/ || m{^\$\(\w+\)} || $_ eq "Makefile" || $_ eq '\\'
+            || $_ eq '@ALLOCA@');
   return $_;
 }
 

gl/m4/non-recursive-gnulib-prefix-hack.m4

@@ -9,6 +9,9 @@ dnl with a value ending in ".h" to reflect that these files are located
 dnl in the directory specified by LIB_DIR.
 AC_DEFUN([gl_NON_RECURSIVE_GNULIB_PREFIX_HACK],
 [
+  # Tell AC_LIBSOURCES where to find source files like alloca.c.
+  AC_CONFIG_LIBOBJ_DIR([lib])
+
   # This hack originated in bison.  It is required when using non-recursive
   # automake rules to build from gnulib-provided lib/ sources.  Hence, LIB_DIR
   # is usually simply "lib".  Those rules use the list of names like "fchdir.o"
@@ -26,4 +29,7 @@ AC_DEFUN([gl_NON_RECURSIVE_GNULIB_PREFIX_HACK],
       (*_H:*.h) eval "$ac_var=$1/\$$ac_var";;
     esac
   done
+
+  # If $ALLOCA is not empty, prefix its value with "lib/".
+  test -n "$ALLOCA" && ALLOCA="lib/$ALLOCA"
 ])