maint: simplify declaration of "libexec" programs

* configure.ac: No need to use 'gl_ADD_PROG' and an indirection
variable '$optional_pkglib_progs' to declare the 'libstdbuf.so'
"libexec" program; the decision to whether compile that program
is not up to the user, but it only and simply depends on whether
the 'stdbuf' "bin" program is to be built or not.

configure.ac

@@ -420,11 +420,11 @@ dnl of coreutils programs to be built only upon explicit user request,
 dnl saving that list in the $no_install_progs_default shell variable.
 m4_include([m4/cu-progs.m4])
 
-# Now that we know which programs will actually be built up, figure out
+# Now that we know which programs will actually be built, determine
 # which optional helper progs should be compiled.
-optional_pkglib_progs=
 case " $optional_bin_progs " in
-  *' stdbuf '*) gl_ADD_PROG([optional_pkglib_progs], [libstdbuf.so]) ;;
+  *' stdbuf '*) pkglibexec_PROGRAMS='src/libstdbuf.so';;
+  *) pkglibexec_PROGRAMS='';;
 esac
 
 man1_MANS=`
@@ -448,9 +448,7 @@ EXTRA_MANS=`for p in $no_install_progs_default; do echo man/$p.1; done`
 # with $(EXEEXT) appending on it, so we have to do it ourselves -- in
 # this case, only for $(bin_PROGRAMS).
 bin_PROGRAMS=`
-  for p in $optional_bin_progs; do echo "src/$p\$(EXEEXT)"; done`
-pkglibexec_PROGRAMS=`
-  for p in $optional_pkglib_progs; do echo src/$p; done`
+  for p in $optional_bin_progs; do echo src/"$p"'$(EXEEXT)'; done`
 
 # Normalize whitespace.
 man1_MANS=`echo $man1_MANS`