Need to install mingw-w64 on linux to cross compile

Fix build on MinGW32

Make compilers without ref-qualifier support happy.

strdup is not part C or C++ standard but POXIS function. Several
compilers failed to compile strdup or generate warnings.

This excludes cygwin and mingw from using the self implemented
functions.

Make no warning test stricter.

- Now it includes -Wextra except for unused-parameter.
- Removed gtest dependency

Make code free of missing-field-initializers warnings

Merge master into 3.4.x

Make .bzl files compatible with future versions of Bazel

* Fix the bug in php c extension that setting one field can change another
field's value.

The reason is that previously, in c extension, it was assumed that the
order that fields were declared in php is the same as the order of
fields in upb. This is not true. Now, for every field in upb, we will
look up the actual property that is corresponding to the upb field.

* Cleanup pull request

* Fix indentation

* Port to php5

* Port with php7.1

* Port to zts

* Add descriptors test

* Update descriptors tests

* Add public descriptors

* Add test_desriptors.proto to test script

* Update composer files

* Remove references to GPBType, update tests to be compatible with c

* Update for c extension compatibility

* Remove nested enums for descriptor, update tests

* Strip leading '.' from descriptor name

* Update tests with test for getClass, fix OneofDescriptor

* Add new files to Makefile.am

Update third party addons with ProfaneDB

I added my project ProfaneDB, it is a database for Protocol Buffers objects. Written in C++, it uses gRPC as an interface for other languages.
It is still work in progress, but I'd love to get some feedback on it while I progress!

Fix invalid offsetof warning.

Fix typo.

Use composition instead of inheritance for oneof default.

Verified test succeed under draconian heap checker

Bump csharp version

Update change log

Merge master into 3.4.x