Fixed port_def/undef so that the amalgamation is self-contained.

tools/amalgamate.py

@@ -18,14 +18,14 @@ class Amalgamator:
 
     self.output_c.write("// Amalgamated source file\n")
     self.output_c.write('#include "upb.h"\n')
-    self.output_c.write('#include "upb/port_def.inc"\n')
+    self.output_c.write(open("upb/port_def.inc").read())
 
     self.output_h.write("// Amalgamated source file\n")
-    self.output_h.write('#include "upb/port_def.inc"\n')
+    self.output_h.write(open("upb/port_def.inc").read())
 
   def finish(self):
-    self.output_c.write('#include "upb/port_undef.inc"\n')
-    self.output_h.write('#include "upb/port_undef.inc"\n')
+    self.output_c.write(open("upb/port_undef.inc").read())
+    self.output_h.write(open("upb/port_undef.inc").read())
 
   def _process_file(self, infile_name, outfile):
     for line in open(infile_name):