Merge branch 'shuujii-define-MRB_NO_GEMS-if-disable_libmruby-is-specified'

lib/mruby/build.rb

@@ -353,11 +353,7 @@ EOS
 
     def define_rules
       compilers.each do |compiler|
-        if respond_to?(:enable_gems?) && enable_gems?
-          compiler.defines -= %w(MRB_NO_GEMS)
-        else
-          compiler.defines += %w(MRB_NO_GEMS)
-        end
+        compiler.defines << "MRB_NO_GEMS" unless enable_gems? && libmruby_enabled?
       end
       [@cc, *(@cxx if cxx_exception_enabled?)].each do |compiler|
         compiler.define_rules(@build_dir, MRUBY_ROOT, @exts.object)