Remove `-std=gnu99` when `enable_cxx_abi`; ref #4703

To stop warnings since C++ do not accept `-std=gnu99` option.

lib/mruby/build.rb

@@ -162,8 +162,8 @@ module MRuby
       compilers.each { |c|
         c.defines += %w(MRB_ENABLE_CXX_EXCEPTION MRB_ENABLE_CXX_ABI)
         c.flags << c.cxx_compile_flag
+        c.flags.delete('-std=gnu99')
       }
-      compilers.each { |c| c.flags << c.cxx_compile_flag }
       linker.command = cxx.command if toolchains.find { |v| v == 'gcc' }
       @cxx_abi_enabled = true
     end