build: provide global defines by `build.defines`.

lib/mruby/build.rb

@@ -71,7 +71,7 @@ module MRuby
 
     include Rake::DSL
     include LoadGems
-    attr_accessor :name, :bins, :exts, :file_separator, :build_dir, :gem_clone_dir
+    attr_accessor :name, :bins, :exts, :file_separator, :build_dir, :gem_clone_dir, :defines
     attr_reader :products, :libmruby_core_objs, :libmruby_objs, :gems, :toolchains, :presym, :mrbc_build, :gem_dir_to_repo_url
 
     alias libmruby libmruby_objs
@@ -97,6 +97,7 @@ module MRuby
         @file_separator = '/'
         @build_dir = "#{build_dir}/#{@name}"
         @gem_clone_dir = "#{build_dir}/repos/#{@name}"
+        @defines = []
         @cc = Command::Compiler.new(self, %w(.c), label: "CC")
         @cxx = Command::Compiler.new(self, %w(.cc .cxx .cpp), label: "CXX")
         @objc = Command::Compiler.new(self, %w(.m), label: "OBJC")

lib/mruby/build/command.rb

@@ -77,7 +77,7 @@ module MRuby
     end
 
     def all_flags(_defines=[], _include_paths=[], _flags=[])
-      define_flags = [defines, _defines].flatten.map{ |d| option_define % d }
+      define_flags = [defines, _defines, build.defines].flatten.map{ |d| option_define % d }
       include_path_flags = [include_paths, _include_paths].flatten.map do |f|
         option_include_path % filename(f)
       end

tasks/mrbgems.rake

@@ -37,7 +37,7 @@ MRuby.each_target do
         f.puts %Q[]
         f.write gem_func_decls
         unless gem_final_calls.empty?
-        f.puts %Q[]
+          f.puts %Q[]
           f.puts %Q[static void]
           f.puts %Q[mrb_final_mrbgems(mrb_state *mrb) {]
           f.write gem_final_calls