Don't pass -lpthread and -lm on Windows

These flags are ignored by cl.exe and have no effect, but add noise to the build logs.

* Reserve unknown fields in upb
1) For decoding, an unknownfields will be lazily created on message,
which contains bytes of unknown fields.
2) For encoding, if the unknownfields is present on message, all bytes
contained in it will be serialized.

* Register the function to encode unknown field at decode time.

* Remove upb_handlers_setaddunknown

* Use upb_sink_putunknown in decoder

* Remove upb_pb_encoder_encode_unknown

* Do not expose encode_unknown

* Implement reserve unknown field in php Implement.

* Make buffer private to CodedInputStream

* Add well known types to php runtime.

* Fix php7.0 tests

* No longer generate empty.proto in test as it has been included in
runtime.

* Fix zts build

* Clean code

* Rename g_p_b_empty to empty.

* Don't generate code for empty.proto in compatibility test

* Fix 32-bit

* Fix mac build

* Fix Makefile.am to add new files

* Fix ruby segment fault

1) rb_ary_new cannot be called during allocate function. During allocate
fucntion, the containing object hasn't been marked and rb_ary_new may
invoke gc to collect containing object.
2) The global map should be marked before allocating it. Otherwise it
may be garbage collected.

* Add test

* Remove commented code

* Fix grammer error

Objc increase test coverage

Add Xcode 9 support to the helper script.

ObjC: Fix merging of length delimited unknown fields.

- Add a test to cover this and tweak the test to not use two merge
  paths to be sure things are as expected.

* Use constexpr more with VC++ 2017

Chrome's official builds have over 170 dynamic initializers for
variables of the form *::TableStruct::aux. Defining
PROTOBUF_CONSTEXPR_VAR to be constexpr for VS 2017 gets rid of all of
these and saves about 10 KB of binary size.

* Update generated_message_table_driven.h

Restore accidentally deleted line.

generate python code when calling PROTOBUF_GENERATE_PYTHON

* Enforce all error report for php tests.

* Import vendor/autoload.php in tests/bootstrap_phpunit.php

protobuf_generate: add EXPORT_MACRO option

Test Shutdown can be called multiple times.

Allow initializing a chain of protos using only a hash in Ruby

* Add $ before url_prefix_len to make it a variable.

* Remove unnecessary string length check

* Remove useless $type_url_len

Fixed to_h with repeated messages to return hashes in Ruby

Conformance Tests for Null Timestamp and Duration JSON Values

Integrated internal changes from Google

Fix go example test.

It needs "-I" flag to find imported file google/protobuf/timestamp.proto

Clang warning cleanup for unused parameter.

Fix policy warning CMP0054

Google.Protobuf should target net45 (upport to upstream/master)

fix protobuf_generate function