Add toolchain support for OpenWRT

tasks/toolchains/openwrt.rake

+# usage of environmental variables to set the
+# cross compiling toolchain proper
+MRuby::Toolchain.new(:openwrt) do |conf|
+  [conf.cc, conf.objc, conf.asm].each do |cc|
+    cc.command = ENV['TARGET_CC']
+    cc.flags = ENV['TARGET_CFLAGS']
+    cc.include_paths = ["#{MRUBY_ROOT}/include"]
+    cc.defines = %w(DISABLE_GEMS)
+    cc.option_include_path = '-I%s'
+    cc.option_define = '-D%s'
+    cc.compile_options = '%{flags} -MMD -o %{outfile} -c %{infile}'
+  end
+
+  [conf.cxx].each do |cxx|
+    cxx.command = ENV['TARGET_CXX']
+    cxx.flags = ENV['TARGET_CXXFLAGS']
+    cxx.include_paths = ["#{MRUBY_ROOT}/include"]
+    cxx.defines = %w(DISABLE_GEMS)
+    cxx.option_include_path = '-I%s'
+    cxx.option_define = '-D%s'
+    cxx.compile_options = '%{flags} -MMD -o %{outfile} -c %{infile}'
+   end
+
+  conf.linker do |linker|
+    linker.command = ENV['TARGET_CC']
+    linker.flags = ENV['TARGET_LDFLAGS']
+    linker.libraries = %w(m)
+    linker.library_paths = []
+    linker.option_library = '-l%s'
+    linker.option_library_path = '-L%s'
+    linker.link_options = '%{flags} -o %{outfile} %{objs} %{flags_before_libraries} %{libs} %{flags_after_libraries}'
+  end
+
+  conf.archiver do |archiver|
+    archiver.command = ENV['TARGET_AR']
+    archiver.archive_options = 'rs %{outfile} %{objs}'
+  end
+end