Modify build script for gcc.

Newly added compile option '-std=gnu99' is not necessary for C++.
And 'g++' should be used to compile for C++ source codes, I think.

tasks/toolchains/gcc.rake

 MRuby::Toolchain.new(:gcc) do |conf|
-  [conf.cc, conf.cxx, conf.objc, conf.asm].each do |cc|
+  [conf.cc, conf.objc, conf.asm].each do |cc|
     cc.command = ENV['CC'] || 'gcc'
     cc.flags = [ENV['CFLAGS'] || %w(-g -std=gnu99 -O3 -Wall -Werror-implicit-function-declaration)]
     cc.include_paths = ["#{MRUBY_ROOT}/include"]
@@ -9,6 +9,16 @@ MRuby::Toolchain.new(:gcc) do |conf|
     cc.compile_options = '%{flags} -MMD -o %{outfile} -c %{infile}'
   end
 
+  [conf.cxx].each do |cxx|
+    cxx.command = ENV['CC'] || 'g++'
+    cxx.flags = [ENV['CFLAGS'] || %w(-g -O3 -Wall -Werror-implicit-function-declaration)]
+    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['LD'] || 'gcc'
     linker.flags = [ENV['LDFLAGS'] || %w()]