Test each combination of boxing, `mrb_int` size, and archtecture.

Boxing:

* `MRB_NO_BOXING`   (`mrb_value` packed in `struct`)
* `MRB_WORD_BOXING` (`mrb_value` packed in `struct`) default
* `MRB_NAN_BOXING`  (`mrb_value` packed in `double`)

`mrb_int` size

* `MRB_INT32`       (`int32_t` as `mrb_int`)
* `MRB_INT64`       (`int64` as `mrb_int`) conflict with `MRB_NAN_BOXING'

Architecture

* `MRB_64BIT`      (`sizeof(void*)` is 64 bits)
* `MRB_32BIT`      (`sizeof(void*)` is 32 bits)

build_config/boxing.rb

-MRuby::Build.new('no-boxing') do |conf|
+MRuby::Build.new('boxing-no-m64-i64') do |conf|
   toolchain :gcc
 
+  conf.cc.flags << '-m64'
+  conf.linker.flags << '-m64'
+
   conf.gembox 'default'
   conf.compilers.each do |c|
-    c.defines += %w(MRB_NO_BOXING)
+    c.defines += %w(MRB_NO_BOXING MRB_INT64)
   end
+  conf.enable_debug
   conf.enable_test
   conf.enable_bintest
 end
 
-MRuby::Build.new('word_boxing') do |conf|
+MRuby::Build.new('boxing-no-m64-i32') do |conf|
   toolchain :gcc
 
+  conf.cc.flags << '-m64'
+  conf.linker.flags << '-m64'
+
   conf.gembox 'default'
   conf.compilers.each do |c|
-    c.defines += %w(MRB_WORD_BOXING)
+    c.defines += %w(MRB_NO_BOXING MRB_INT32)
   end
+  conf.enable_debug
   conf.enable_test
   conf.enable_bintest
 end
 
-MRuby::Build.new('nan_boxing') do |conf|
+MRuby::Build.new('boxing-no-m32-i64') do |conf|
   toolchain :gcc
 
+  conf.cc.flags << '-m32'
+  conf.linker.flags << '-m32'
+
+  # Turn on `enable_debug` for better debugging
+  conf.gembox 'default'
+  conf.compilers.each do |c|
+    c.defines += %w(MRB_NO_BOXING MRB_INT64)
+  end
+  conf.enable_debug
+  conf.enable_test
+  conf.enable_bintest
+end
+
+MRuby::Build.new('boxing-no-m32-i32') do |conf|
+  toolchain :gcc
+
+  conf.cc.flags << '-m32'
+  conf.linker.flags << '-m32'
+
+  # Turn on `enable_debug` for better debugging
+  conf.gembox 'default'
+  conf.compilers.each do |c|
+    c.defines += %w(MRB_NO_BOXING MRB_INT32)
+  end
+  conf.enable_debug
+  conf.enable_test
+  conf.enable_bintest
+end
+
+MRuby::Build.new('boxing-word-m64-i64') do |conf|
+  toolchain :gcc
+
+  conf.cc.flags << '-m64'
+  conf.linker.flags << '-m64'
+
+  conf.gembox 'default'
+  conf.compilers.each do |c|
+    c.defines += %w(MRB_WORD_BOXING MRB_INT64)
+  end
+  conf.enable_debug
+  conf.enable_test
+  conf.enable_bintest
+end
+
+MRuby::Build.new('boxing-word-m64-i32') do |conf|
+  toolchain :gcc
+
+  conf.cc.flags << '-m64'
+  conf.linker.flags << '-m64'
+
+  conf.gembox 'default'
+  conf.compilers.each do |c|
+    c.defines += %w(MRB_WORD_BOXING MRB_INT32)
+  end
+  conf.enable_debug
+  conf.enable_test
+  conf.enable_bintest
+end
+
+MRuby::Build.new('boxing-word-m32-i64') do |conf|
+  toolchain :gcc
+
+  conf.cc.flags << '-m32'
+  conf.linker.flags << '-m32'
+
+  conf.gembox 'default'
+  conf.compilers.each do |c|
+    c.defines += %w(MRB_WORD_BOXING MRB_INT64)
+  end
+  conf.enable_debug
+  conf.enable_test
+  conf.enable_bintest
+end
+
+MRuby::Build.new('boxing-word-m32-i32') do |conf|
+  toolchain :gcc
+
+  conf.cc.flags << '-m32'
+  conf.linker.flags << '-m32'
+
+  conf.gembox 'default'
+  conf.compilers.each do |c|
+    c.defines += %w(MRB_WORD_BOXING MRB_INT32)
+  end
+  conf.enable_debug
+  conf.enable_test
+  conf.enable_bintest
+end
+
+MRuby::Build.new('boxing-nan-m64') do |conf|
+  toolchain :gcc
+
+  conf.cc.flags << '-m64'
+  conf.linker.flags << '-m64'
+
   conf.gembox 'default'
   conf.compilers.each do |c|
     c.defines += %w(MRB_NAN_BOXING)
   end
+  conf.enable_debug
+  conf.enable_test
+  conf.enable_bintest
+end
+
+MRuby::Build.new('boxing-nan-m64') do |conf|
+  toolchain :gcc
+
+  conf.cc.flags << '-m32'
+  conf.linker.flags << '-m32'
+
+  conf.gembox 'default'
+  conf.compilers.each do |c|
+    c.defines += %w(MRB_NAN_BOXING MRB_INT32)
+  end
+  conf.enable_debug
   conf.enable_test
   conf.enable_bintest
 end