'make check' fails on Ubuntu 12.04

[tseaver]

due to use of INT32_MAX / INT32_MIN, which are non-standard under C99.

6ef984a#commitcomment-8546305

[tseaver]

Failed build:

g++ -DHAVE_CONFIG_H -I. -I..  -I../gtest/include -I../gtest/include  -pthread -Wall -Wwrite-strings -Woverloaded-virtual -Wno-sign-compare -g -DNDEBUG -MT google/protobuf/protobuf_test-drop_unknown_fields_test.o -MD -MP -MF google/protobuf/.deps/protobuf_test-drop_unknown_fields_test.Tpo -c -o google/protobuf/protobuf_test-drop_unknown_fields_test.o `test -f 'google/protobuf/drop_unknown_fields_test.cc' || echo './'`google/protobuf/drop_unknown_fields_test.cc
In file included from google/protobuf/drop_unknown_fields_test.cc:36:0:
./google/protobuf/unittest_drop_unknown_fields.pb.h:49:64: error: ‘INT32_MIN’ was not declared in this scope
./google/protobuf/unittest_drop_unknown_fields.pb.h:50:64: error: ‘INT32_MAX’ was not declared in this scope
./google/protobuf/unittest_drop_unknown_fields.pb.h:72:94: error: ‘INT32_MIN’ was not declared in this scope
./google/protobuf/unittest_drop_unknown_fields.pb.h:73:94: error: ‘INT32_MAX’ was not declared in this scope

[tseaver]

This patch fixes make check for me

Note that the two header files are checked-in even though generated: they needed to be re-generated after updating the code-generator.