require not working with mruby

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I try to create an executable from a simple Ruby program. If the file contains a simple puts with a string and I use the following commands
mrbc -Btest load.rb
gcc -std=c99 -I/root/mruby/include -I. test_stub.c -o load /root/mruby/build/host/lib/libmruby.a -lm -ldl
everything runs well calling ./load display my string.
But if I add
require 'yaml' or require 'ostruct' or require 'json' before, execution simply display nothig, no crash either...
I have added the following line in mrbgems/default.gembox file
conf.gem :core => "mruby-yaml"

and
conf.gem :github => 'ksss/mruby-ostruct', :branch => 'master'
conf.gem :github => 'mattn/mruby-json'
conf.gem :github => 'mattn/mruby-require'
in build_config.rb after conf.gembox 'default'
After make clean a call to objdump on libmruby.a show that the modules are in the lib...
But it doesn't work!
What am I doing wrong ?

Start by reviewing mrbgems/default.gembox, build_config.rb, and test_stub.c, then reproduce the shown mrbc and gcc commands with the reported require cases. Compare the resulting libmruby.a and executable behavior; done means ./load still displays the string when yaml, ostruct, or json is required.

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