don't always generate gem functions

If the src and mrblib directories of a mrbgem don't exist or don't
include "usable" files (for tools like mirb), then functions for gem
initialization and finalization don't have to be generated.

tasks/mrbgem_spec.rake

@@ -36,6 +36,8 @@ module MRuby
 
       attr_accessor :export_include_paths
 
+      attr_reader :generate_functions
+
       attr_block MRuby::Build::COMMANDS
 
       def initialize(name, &block)
@@ -56,7 +58,9 @@ module MRuby
         @objs = Dir.glob("#{dir}/src/*.{c,cpp,cxx,cc,m,asm,s,S}").map do |f|
           objfile(f.relative_path_from(@dir).to_s.pathmap("#{build_dir}/%X"))
         end
-        @objs << objfile("#{build_dir}/gem_init")
+
+        @generate_functions = !(@rbfiles.empty? && @objs.empty?)
+        @objs << objfile("#{build_dir}/gem_init") if @generate_functions
 
         @test_rbfiles = Dir.glob("#{dir}/test/*.rb")
         @test_objs = Dir.glob("#{dir}/test/*.{c,cpp,cxx,cc,m,asm,s,S}").map do |f|
@@ -89,7 +93,7 @@ module MRuby
           compiler.include_paths << "#{dir}/include" if File.directory? "#{dir}/include"
         end
 
-        define_gem_init_builder
+        define_gem_init_builder if @generate_functions
       end
 
       def add_dependency(name, *requirements)

tasks/mrbgems.rake

@@ -10,6 +10,17 @@ MRuby.each_target do
     file "#{build_dir}/mrbgems/gem_init.c" => [MRUBY_CONFIG, __FILE__] do |t|
       FileUtils.mkdir_p "#{build_dir}/mrbgems"
       open(t.name, 'w') do |f|
+        gem_func_gems = gems.select { |g| g.generate_functions }
+        gem_func_decls = gem_func_gems.each_with_object('') do |g, s|
+          s << "void GENERATED_TMP_mrb_#{g.funcname}_gem_init(mrb_state*);\n" \
+               "void GENERATED_TMP_mrb_#{g.funcname}_gem_final(mrb_state*);\n"
+        end
+        gem_init_calls = gem_func_gems.each_with_object('') do |g, s|
+          s << "  GENERATED_TMP_mrb_#{g.funcname}_gem_init(mrb);\n"
+        end
+        gem_final_calls = gem_func_gems.each_with_object('') do |g, s|
+          s << "  GENERATED_TMP_mrb_#{g.funcname}_gem_final(mrb);\n"
+        end
         f.puts %Q[/*]
         f.puts %Q[ * This file contains a list of all]
         f.puts %Q[ * initializing methods which are]
@@ -22,19 +33,17 @@ MRuby.each_target do
         f.puts %Q[]
         f.puts %Q[#include "mruby.h"]
         f.puts %Q[]
-        f.puts %Q[#{gems.map{|g| "void GENERATED_TMP_mrb_%s_gem_init(mrb_state* mrb);" % g.funcname}.join("\n")}]
-        f.puts %Q[]
-        f.puts %Q[#{gems.map{|g| "void GENERATED_TMP_mrb_%s_gem_final(mrb_state* mrb);" % g.funcname}.join("\n")}]
+        f.write gem_func_decls
         f.puts %Q[]
         f.puts %Q[static void]
         f.puts %Q[mrb_final_mrbgems(mrb_state *mrb) {]
-        f.puts %Q[#{gems.map{|g| "GENERATED_TMP_mrb_%s_gem_final(mrb);" % g.funcname}.join("\n")}]
+        f.write gem_final_calls
         f.puts %Q[}]
         f.puts %Q[]
         f.puts %Q[void]
         f.puts %Q[mrb_init_mrbgems(mrb_state *mrb) {]
-        f.puts %Q[#{gems.map{|g| "GENERATED_TMP_mrb_%s_gem_init(mrb);" % g.funcname}.join("\n")}]
-        f.puts %Q[mrb_state_atexit(mrb, mrb_final_mrbgems);]
+        f.write gem_init_calls
+        f.puts %Q[  mrb_state_atexit(mrb, mrb_final_mrbgems);] unless gem_final_calls.empty?
         f.puts %Q[}]
       end
     end

tasks/ruby_ext.rake

@@ -42,6 +42,15 @@ class Symbol
   end
 end
 
+module Enumerable
+  # Compatible with 1.9 on 1.8
+  def each_with_object(memo)
+    return to_enum :each_with_object, memo unless block_given?
+    each { |obj| yield obj, memo }
+    memo
+  end
+end
+
 $pp_show = true
 
 if $verbose.nil?