Remove HAVE_GCC_SYNC flag

Author: ianks

ext/concurrent/extconf.rb

@@ -43,13 +43,6 @@ def compiler_is_gcc
       end
     end
 
-    try_run(<<CODE,$CFLAGS) && ($defs << '-DHAVE_GCC_SYNC')
-      int main() {
-        __sync_synchronize();
-        return 0;
-      }
-CODE
-
     create_makefile('concurrent/' + EXTENSION_NAME)
   rescue
     create_dummy_makefile