[ruby/json] Compile with std=c99

https://github.com/ruby/json/commit/d4968d2e48

ext/json/generator/extconf.rb

@@ -4,6 +4,7 @@ if RUBY_ENGINE == 'truffleruby'
   # The pure-Ruby generator is faster on TruffleRuby, so skip compiling the generator extension
   File.write('Makefile', dummy_makefile("").join)
 else
+  append_cflags("-std=c99")
   $defs << "-DJSON_GENERATOR"
   create_makefile 'json/ext/generator'
 end

ext/json/generator/generator.c

@@ -257,7 +257,7 @@ static void convert_UTF8_to_ASCII_only_JSON(FBuffer *out_buffer, VALUE str, cons
                     }
 
                     for (short i = 1; i < ch_len; i++) {
-                        wchar = (wchar<<6) | (ptr[pos+i] & 0x3F);
+                        wchar = (wchar << 6) | (ptr[pos+i] & 0x3F);
                     }
 
                     FLUSH_POS(ch_len);

ext/json/parser/extconf.rb

@@ -29,4 +29,6 @@ rescue NoMethodError
   $CFLAGS << ' -DSTR_UMINUS_DEDUPE_FROZEN=0 '
 end
 
+append_cflags("-std=c99")
+
 create_makefile 'json/ext/parser'