Make it possible that `libmruby.a` is not created

Previously, `libmruby.a` was created even if only `mruby-bin-mrbc` or`
mruby-compiler` was specified for gem, but by specifying `disable_libmruby`,
the creation of `libmruby.a` can be suppressed.

### Note

The https://github.com/mruby/mruby/pull/5084#issuecomment-723521971
incompatibility seems to be difficult for users to avoid, so the original
behavior has been restored. Therefore, if we need `mrbc` other than the
`host` build, we need to explicitly specify `mruby-bin-mrbc` gem.

Due to the above changes, `build_config/boxing.rb` etc. will not work, but I
have added` mruby-bin-mrbc` to `default.gembox` to fix it. I don't think
this change is a big deal because originally `mruby-compiler` was included.

3 files changed, 17 insertions, 20 deletions

diff --git a/mrbgems/default.gembox b/mrbgems/default.gembox
index de169906b..ca8938e87 100644
--- a/mrbgems/default.gembox
+++ b/mrbgems/default.gembox
@@ -78,6 +78,9 @@ MRuby::GemBox.new do |conf|
   conf.gem :core => "mruby-rational"
   conf.gem :core => "mruby-complex"
 
+  # Generate mrbc command
+  conf.gem :core => "mruby-bin-mrbc"
+
   # Generate mirb command
   conf.gem :core => "mruby-bin-mirb"
 
diff --git a/mrbgems/mruby-bin-mrbc/mrbgem.rake b/mrbgems/mruby-bin-mrbc/mrbgem.rake
index 48b67aedb..67200b3f0 100644
--- a/mrbgems/mruby-bin-mrbc/mrbgem.rake
+++ b/mrbgems/mruby-bin-mrbc/mrbgem.rake
@@ -2,15 +2,14 @@ MRuby::Gem::Specification.new 'mruby-bin-mrbc' do |spec|
   spec.license = 'MIT'
   spec.author  = 'mruby developers'
   spec.summary = 'mruby compiler executable'
-
   spec.add_dependency 'mruby-compiler', :core => 'mruby-compiler'
 
   exec = exefile("#{build.build_dir}/bin/mrbc")
-  mrbc_objs = Dir.glob("#{spec.dir}/tools/mrbc/*.c").map { |f| objfile(f.pathmap("#{spec.build_dir}/tools/mrbc/%n")) }.flatten
+  mrbc_objs = Dir.glob("#{spec.dir}/tools/mrbc/*.c").map { |f| objfile(f.pathmap("#{spec.build_dir}/tools/mrbc/%n")) }
 
-  file exec => mrbc_objs + [build.libmruby_core_static] do |t|
+  file exec => mrbc_objs << build.libmruby_core_static do |t|
     build.linker.run t.name, t.prerequisites
   end
 
-  build.bins << 'mrbc' unless build.bins.find { |v| v == 'mrbc' }
+  build.bins << 'mrbc'
 end
diff --git a/mrbgems/mruby-compiler/mrbgem.rake b/mrbgems/mruby-compiler/mrbgem.rake
index 9df846abb..70f9e19d8 100644
--- a/mrbgems/mruby-compiler/mrbgem.rake
+++ b/mrbgems/mruby-compiler/mrbgem.rake
@@ -3,20 +3,18 @@ MRuby::Gem::Specification.new 'mruby-compiler' do |spec|
   spec.author  = 'mruby developers'
   spec.summary = 'mruby compiler library'
 
-  lex_def = "#{dir}/core/lex.def"
-  core_objs = Dir.glob("#{dir}/core/*.c").map { |f|
-    next nil if build.cxx_exception_enabled? and f =~ /(codegen|y\.tab)\.c$/
-    objfile(f.pathmap("#{build_dir}/core/%n"))
-  }.compact
-
-  if build.cxx_exception_enabled?
-    core_objs <<
-      build.compile_as_cxx("#{dir}/core/y.tab.c", "#{build_dir}/core/y.tab.cxx",
-                           objfile("#{build_dir}/y.tab"), ["#{dir}/core"]) <<
-      build.compile_as_cxx("#{dir}/core/codegen.c", "#{build_dir}/core/codegen.cxx")
-  else
-    core_objs << objfile("#{build_dir}/core/y.tab")
+  as_cxx_srcs = %w[codegen y.tab].map{|name| "#{dir}/core/#{name}.c"}
+  objs = Dir.glob("#{dir}/core/*.c").map do |src|
+    dst = src.pathmap("#{build_dir}/core/%n")
+    if build.cxx_exception_enabled? && as_cxx_srcs.include?(src)
+      build.compile_as_cxx(src, "#{dst}.cxx")
+    else
+      objfile(dst)
+    end
   end
+  build.libmruby_core_objs << objs
+
+  lex_def = "#{dir}/core/lex.def"
 
   # Parser
   file "#{dir}/core/y.tab.c" => ["#{dir}/core/parse.y", lex_def] do |t|
@@ -30,9 +28,6 @@ MRuby::Gem::Specification.new 'mruby-compiler' do |spec|
     replace_line_directive(t.name)
   end
 
-  file build.libmruby_core_static => core_objs
-  build.libmruby << core_objs
-
   def replace_line_directive(path)
     content = File.read(path).gsub(%r{
       ^\#line\s+\d+\s+"\K.*$ |                  # #line directive