Add weak dependency feature for mrbgems

[dearblue]

This feature is a hint to change the mrbgem inclusion order when specified by spec.add_dependency of mrbgem.rake.
It does not mean that it is depends directly, so it will not include mrbgem by itself.

To use this feature, give the weak: true argument to the MRuby::Gem::Specification#add_dependency method in mruby-XXX/mrbgem.rake.

spec.add_dependency "mruby-sprintf", :weak => true

Example with mruby-print and mruby-sprintf:

• my_build_config.rb

  configurations = {
    build: {
      host: {
        gems: [
          { core: "mruby-sprintf" },
          { core: "mruby-print" },
        ]
      },
  
      ### So far in the wrong mrbgems order
      host2: {
        gems: [
          { core: "mruby-print" },
          { core: "mruby-sprintf" }
        ]
      },
  
      ### Not including mruby-sprintf
      host3: {
        gems: [
          { core: "mruby-print" }
        ]
      },
  
      host4: {
        gems: [
          { core: "mruby-sprintf" }
        ]
      }
    }
  }
  
  configurations[:build].each do |name, conf|
    MRuby::Build.new(name.to_s) do |c|
      toolchain :gcc
      conf[:gems].each { |*g| gem *g }
    end
  end

• Build summary of MRUBY_CONFIG=my_build_config.rb ./minirake clean all

  ================================================
        Config Name: host
   Output Directory: build/host
           Binaries: mrbc
      Included Gems:
               mruby-sprintf - standard Kernel#sprintf method
               mruby-print - standard print/puts/p
               mruby-compiler - mruby compiler library
               mruby-bin-mrbc - mruby compiler executable
  ================================================
  
  ### mruby-sprintf is weak dependency, so included mruby-sprintf before mruby-print
  ================================================
        Config Name: host2
   Output Directory: build/host2
      Included Gems:
               mruby-sprintf - standard Kernel#sprintf method
               mruby-print - standard print/puts/p
  ================================================
  
  ### mruby-sprintf is weak dependency, but it is not specified directly, so included mruby-print only
  ================================================
        Config Name: host3
   Output Directory: build/host3
      Included Gems:
               mruby-print - standard print/puts/p
  ================================================
  
  ================================================
        Config Name: host4
   Output Directory: build/host4
      Included Gems:
               mruby-sprintf - standard Kernel#sprintf method
  ================================================
  

Compatible problems for users

It is not backwards compatible. Also it will always be treated as dependent for previous mruby versions.

Therefore, it is necessary to decide by require "mruby/source" and MRuby::Source::MRUBY_RELEASE_NO.

e.g.:

# A part of mruby-XXX/mrbgem.rake

require "mruby/source"

spec.add_dependency "mruby-YYY", weak: true if MRuby::Source::MRUBY_RELEASE_NO >= 20000

[dearblue]

This pull request will be withdrawn for the following reasons:

• Ruby allows undefined classes and methods to be written first.
  For example, the Regexp or Complex class, or the sprintf in the printf method.
• Weak dependency becomes unnecessary by cutting out the dependent part and creating another mrbgem and defining a clear dependency.