build system: always enable presym for mrbc build

Remove disable_presym from mrbc_build and cross-build fallback.
Add explicit object file dependencies on presym headers in
presym.rake to prevent compilation before headers are generated.

Co-authored-by: Claude <noreply@anthropic.com>

build_config/mrbc.rb

@@ -7,5 +7,4 @@ MRuby::Build.new do |conf|
 
   conf.build_mrbc_exec
   conf.disable_libmruby
-  conf.disable_presym
 end

lib/mruby/build.rb

@@ -156,11 +156,7 @@ module MRuby
         current.instance_eval(&block)
       ensure
         if current.libmruby_enabled? && !current.mrbcfile_external?
-          if current.presym_enabled?
-            current.create_mrbc_build if current.host? || current.gems["mruby-bin-mrbc"]
-          elsif current.host?
-            current.build_mrbc_exec
-          end
+          current.create_mrbc_build if current.host? || current.gems["mruby-bin-mrbc"]
         end
         current.presym = Presym.new(current) if current.presym_enabled?
       end
@@ -192,13 +188,6 @@ module MRuby
       @enable_presym
     end
 
-    def disable_presym
-      if @enable_presym
-        @enable_presym = false
-        compilers.each{|c| c.defines << "MRB_NO_PRESYM"}
-      end
-    end
-
     def disable_lock
       @enable_lock = false
     end
@@ -569,7 +558,7 @@ EOS
       end
       build.build_mrbc_exec
       build.disable_libmruby
-      build.disable_presym
+      build.presym = Presym.new(build) if build.presym_enabled?
       @mrbc_build = build
       self.mrbcfile = build.mrbcfile
       build
@@ -592,7 +581,6 @@ EOS
           conf.toolchain
           conf.build_mrbc_exec
           conf.disable_libmruby
-          conf.disable_presym
         end
       end
     end

tasks/presym.rake

@@ -44,5 +44,15 @@ MRuby.each_target do |build|
     end
   end
 
+  # Ensure object files depend on presym headers being generated.
+  # This is necessary for sub-builds (e.g., mrbc) whose compilation
+  # may be triggered during another build's presym scanning.
+  prereqs.each_key do |prereq|
+    next unless File.extname(prereq) == build.exts.object
+    next unless prereq.start_with?(build_dir)
+    next if mrbc_build_dir && prereq.start_with?(mrbc_build_dir)
+    file prereq => presym.list_path
+  end
+
   gensym_task.enhance([presym.list_path])
 end