Separate compilers setup from config setup

Fixes #3418.

tasks/mrbgem_spec.rake

@@ -90,7 +90,9 @@ module MRuby
         build.libmruby << @objs
 
         instance_eval(&@build_config_initializer) if @build_config_initializer
+      end
 
+      def setup_compilers
         compilers.each do |compiler|
           compiler.define_rules build_dir, "#{dir}"
           compiler.defines << %Q[MRBGEM_#{funcname.upcase}_VERSION=#{version}]
@@ -419,6 +421,8 @@ module MRuby
 
         @ary = tsort_dependencies gem_table.keys, gem_table, true
 
+        each(&:setup_compilers)
+
         each do |g|
           import_include_paths(g)
         end