Use `Dir.glob` instead of `map`.
MODIFY
lib/mruby/build/load_gems.rb
lib/mruby/build/load_gems.rb
@@ -35,9 +35,7 @@ module MRuby
current.build_config_initializer = block
gems << current
- cxx_srcs = ['src', 'test', 'tools'].map do |subdir|
- Dir.glob("#{current.dir}/#{subdir}/*.{cpp,cxx,cc}")
- end.flatten
+ cxx_srcs = Dir.glob("#{current.dir}/{src,test,tools}/*.{cpp,cxx,cc}")
enable_cxx_exception unless cxx_srcs.empty?
current

Use `Dir.glob` instead of `map`.