Replace C++11 only method std::map::at

Update version number to 3.5.1

`umask 0022` makes sure the created package will be accessible by all users.

Avoid calling method from php extension directly

Create containing directory before generating well_known_types_embed.cc

Remove py2.6 support.

available.

Use matching enum type for IsPOD.

Explicitly propagate the status of Flush().

Before the change, an implicit Flush() will be triggered in the
destructor of the input stream. However, the return code of Flush() is
not discarded. This change makes sure when Flush() fails, we will
return false.

This fixes the following build error:

oldpwd=`pwd` && cd .../protobuf/src && \
$oldpwd/js_embed google/protobuf/compiler/js/well_known_types/any.js google/protobuf/compiler/js/well_known_types/struct.js google/protobuf/compiler/js/well_known_types/timestamp.js > $oldpwd/google/protobuf/compiler/js/well_known_types_embed.cc
/bin/bash: line 1: .../protobuf/target/src/google/protobuf/compiler/js/well_known_types_embed.cc: No such file or directory
Makefile:8201: recipe for target 'google/protobuf/compiler/js/well_known_types_embed.cc' failed

which is observed during the cross-compilation since the version 3.2.

This changes the linking error if php extension is not statically linked
to a runtime error. In this way, users who don't need Timestamp can
still use protobuf even if date extension is not statically linked in
php.

Add backslach to make class explict in global namespace

can be used at install time.

Merge 3.5.0.1 to 3.5.x

* Fix ruby gc_test in ruby 2.4

* Initialize global variables to Qnil.

* Add discard unknown API in ruby.

* Add test for oneof message field.

* Add TestUnknown to represent unknown field data clearly.

* Only serialize the message with unknown fields itself in test.

* Move discard_unknown from Message to Google.Protobuf

* Recursively clear unknown fields in submessages.

* Recursively discard unknown fields in submsg for c extension

* Fix zts build

* Add comment for tests

* Add a TODO to add a util for encoding varint for better readability.

* Add test for oneof message field.

io_win32: support non-ASCII paths

Unlike GetEnvironmentVariableW,
GetCurrentDirectoryW doesn't set
ERROR_INSUFFICIENT_BUFFER.

If the test cannot find a temp directory by
checking environment variables, it will fall back
to using the current working directory as the temp
directory root.

This is what the test used to do as of commit
https://github.com/google/protobuf/commit/6de51caed52d798815954646b230c5aef3e4d2fc
and what was then changed by commit
https://github.com/google/protobuf/pull/3978/commits/792d098769d8e000d8d474c8ffd201d2eabc2134

Fixes https://github.com/google/protobuf/issues/3951

Do not use "googletest.h", apprently that leads to
linking errors on Windows which I couldn't figure
out how to solve, and decided to just go with
plain gTest instead.

See https://github.com/google/protobuf/issues/3951