available.

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.

can be used at install time.

* 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.

* Provide discardUnknownFields API in php implementation

* Provide discardUnknownFields API in php c extension.

* Fix php well known type conformance tests

* Properly generate code for test.proto

* Provide GPBMetadata files in c extensions for generated files to import.

* Remove unnecessary test

* Clean up code

* Add declaration for initOnce.

* Refactoring

* Fix php well known type conformance tests

* Properly generate code for test.proto

* Provide GPBMetadata files in c extensions for generated files to import.

* Remove unnecessary test

* Clean up code

* Add declaration for initOnce.

* Refactoring

* 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

* Add php support for Timestamp.

* Fix comments

* Add any support in php runtime.

* Remove unused file in config.m4

* Fix comments

* Fix error for tsrmls build

* Add newly added file to Makefile.am

* 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

Both native_slot_merge and native_slot_merge_by_array have no return
type declared. GCC implicitly uses int as return type in such cases,
however, these functions are actually void.

In the generated code of previous versions, each php field is given an
initial value. In c extension, it was assumed that the field order in
the generated code is consistent with upb fields order, so that the
correct initial value can be bound to the correct upb field. However,
this may not be true. The order of fields in generated code is decided
by proto compiler, while the order of upb fields is decided by the hash
function used in c extension.
This PR fixes the issue by reset the initial value at runtime.