Fixed language standard mode for GCC (cannot use gnu99 with C++, c++03 used instead)

tasks/toolchains/gcc.rake

@@ -6,7 +6,7 @@ MRuby::Toolchain.new(:gcc) do |conf, _params|
     cc.option_include_path = '-I%s'
     cc.option_define = '-D%s'
     cc.compile_options = '%{flags} -MMD -o %{outfile} -c %{infile}'
-    cc.cxx_compile_flag = '-x c++'
+    cc.cxx_compile_flag = '-x c++ -std=c++03'
   end
 
   [conf.cxx].each do |cxx|
@@ -16,7 +16,7 @@ MRuby::Toolchain.new(:gcc) do |conf, _params|
     cxx.option_include_path = '-I%s'
     cxx.option_define = '-D%s'
     cxx.compile_options = '%{flags} -MMD -o %{outfile} -c %{infile}'
-    cxx.cxx_compile_flag = '-x c++'
+    cxx.cxx_compile_flag = '-x c++ -std=c++03'
   end
 
   conf.linker do |linker|