Merge pull request #2379 from monaka/pr-do-not-export-include-dir-in-gems-to-core

Do not export include/ dirs in mrbgems to core build.

tasks/mrbgem_spec.rake

@@ -49,9 +49,6 @@ module MRuby
 
       def setup
         MRuby::Gem.current = self
-        @build.compilers.each do |compiler|
-          compiler.include_paths << "#{dir}/include"
-        end if File.directory? "#{dir}/include"
         MRuby::Build::COMMANDS.each do |command|
           instance_variable_set("@#{command}", @build.send(command).clone)
         end
@@ -88,6 +85,7 @@ module MRuby
         compilers.each do |compiler|
           compiler.define_rules build_dir, "#{dir}"
           compiler.defines << %Q[MRBGEM_#{funcname.upcase}_VERSION=#{version}]
+          compiler.include_paths << "#{dir}/include" if File.directory? "#{dir}/include"
         end
 
         define_gem_init_builder