Add `host-shared.rb` to create shared mruby library.

target/host-shared.rb

+MRuby::Build.new do |conf|
+  # load specific toolchain settings
+
+  # Gets set by the VS command prompts.
+  if ENV['VisualStudioVersion'] || ENV['VSINSTALLDIR']
+    toolchain :visualcpp
+  else
+    toolchain :gcc
+  end
+
+  # include the GEM box
+  conf.gembox 'full-core'
+
+  # C compiler settings
+  conf.cc do |cc|
+    cc.flags = '-fPIC'
+  end
+
+  # mrbc settings
+  # conf.mrbc do |mrbc|
+  #   mrbc.compile_options = "-g -B%{funcname} -o-" # The -g option is required for line numbers
+  # end
+
+  conf.archiver do |archiver|
+    archiver.command = 'gcc'
+    archiver.archive_options = '-shared -o %{outfile} %{objs}'
+  end
+
+  # Linker settings
+  # conf.linker do |linker|
+  #   linker.command = ENV['LD'] || 'gcc'
+  #   linker.flags = [ENV['LDFLAGS'] || []]
+  #   linker.flags_before_libraries = []
+  #   linker.libraries = %w()
+  #   linker.flags_after_libraries = []
+  #   linker.library_paths = []
+  #   linker.option_library = '-l%s'
+  #   linker.option_library_path = '-L%s'
+  #   linker.link_options = "%{flags} -o "%{outfile}" %{objs} %{libs}"
+  # end
+
+  # Archiver settings
+  # conf.archiver do |archiver|
+  #   archiver.command = ENV['AR'] || 'ar'
+  #   archiver.archive_options = 'rs "%{outfile}" %{objs}'
+  # end
+
+  # Parser generator settings
+  # conf.yacc do |yacc|
+  #   yacc.command = ENV['YACC'] || 'bison'
+  #   yacc.compile_options = %q[-o "%{outfile}" "%{infile}"]
+  # end
+
+  # gperf settings
+  # conf.gperf do |gperf|
+  #   gperf.command = 'gperf'
+  #   gperf.compile_options = %q[-L ANSI-C -C -p -j1 -i 1 -g -o -t -N mrb_reserved_word -k"1,3,$" "%{infile}" > "%{outfile}"]
+  # end
+
+  # file extensions
+  conf.exts do |exts|
+     exts.library = '.so'
+  end
+
+  # file separetor
+  # conf.file_separator = '/'
+
+  # Turn on `enable_debug` for better debugging
+  enable_debug
+  conf.enable_bintest
+  conf.enable_test
+end