[Bug #19005] dynamic_lookup linker option in external libraries

The warning against `-undefined dynamic_lookup` is just a warning yet,
and many gems seem to pay no attention to warnings.  Until it fails
actually, keep it as a migration path, except for standard extension
libraries and bundled extension gems.

configure.ac

@@ -2773,14 +2773,26 @@ AS_IF([test "$with_dln_a_out" != yes], [
   AS_IF([test "$rb_cv_dlopen" = yes], [
     AS_CASE(["$target_os"],
       [darwin*], [
+        AC_SUBST(ADDITIONAL_DLDFLAGS, "")
 	for flag in \
 	  "-multiply_defined suppress" \
+	  "-undefined dynamic_lookup" \
 	  ; do
-	  test "x${linker_flag}" = x || flag="${linker_flag}`echo ${flag} | tr ' ' ,`"
-	  RUBY_TRY_LDFLAGS([$flag], [], [flag=])
-	  AS_IF([test "x$flag" != x], [
-	    RUBY_APPEND_OPTIONS(DLDFLAGS, [$flag])
-	  ])
+      test "x${linker_flag}" = x || flag="${linker_flag}`echo ${flag} | tr ' ' ,`"
+      RUBY_TRY_LDFLAGS([$flag], [], [$flag=])
+      AS_IF([test x"$flag" = x], [continue])
+
+      AC_MSG_CHECKING([whether $flag is accepted for bundle])
+      : > conftest.c
+      AS_IF([${LDSHARED/'$(CC)'/$CC} -o conftest.bundle $flag conftest.c >/dev/null 2>conftest.err &&
+          test ! -s conftest.err], [
+          AC_MSG_RESULT([yes])
+          RUBY_APPEND_OPTIONS(DLDFLAGS, [$flag])
+      ], [
+          AC_MSG_RESULT([no])
+          RUBY_APPEND_OPTIONS(ADDITIONAL_DLDFLAGS, [$flag])
+      ])
+      rm -fr conftest.*
 	done
       ])
   ])

lib/mkmf.rb

@@ -2557,6 +2557,7 @@ site-install-rb: install-rb
     $INCFLAGS << " -I$(hdrdir)/ruby/backward" unless $extmk
     $INCFLAGS << " -I$(hdrdir) -I$(srcdir)"
     $DLDFLAGS = with_config("dldflags", arg_config("DLDFLAGS", config["DLDFLAGS"])).dup
+    config_string("ADDITIONAL_DLDFLAGS") {|flags| $DLDFLAGS << " " << flags} unless $extmk
     $LIBEXT = config['LIBEXT'].dup
     $OBJEXT = config["OBJEXT"].dup
     $EXEEXT = config["EXEEXT"].dup