Fixed cross compilations with the Autotools build

Minor fix for autocreated object repeated fields and maps.

- If setting/clearing a repeated field/map that was objects, check the class
  before checking the autocreator.
- Just to be paranoid, don’t mutate within copy/mutableCopy for the autocreated
  classes to ensure there is less chance of issues if someone does something
  really crazy threading wise.
- Some more tests for the internal AutocreatedArray/AutocreatedDictionary
  classes to ensure things are working as expected.
- Add Xcode 8.2 to the full_mac_build.sh supported list.

Add link to Elm proto plugin

Fix include in auto-generated well_known_types_embed.cc

Removed arena_nc.cc and arena_nc_test.py

This test is undocumented and it looks like it has probably never
worked. Let's just remove it to tidy things up. This fixes issue #2515.

Pull request #2517 caused cross compilations to start failing, because
the js_embed binary was being built to run on the target platform
instead of on the build machine. This change updates the Autotools build
to use the AX_PROG_CXX_FOR_BUILD macro to find a suitable compiler for
the build machine and always use that when building js_embed.

Fix warning in compiler/js/embed.cc

Restore include style fix (e3da722b) that has been trampled by
auto-generation of well_known_types_embed.cc

embed.cc: In function ‘std::string CEscape(const string&)’:
embed.cc:51:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (int i = 0; i < str.size(); ++i) {
                                ^

Init index_in_metadata_ without condition

Eliminate redundant methods in C++ generated code for lite protos

Chromium MemorySanitizer (MSan) reports use-of-uninitialized-value
of index_in_metadata_ attribute from EnumGenerator class. Fix these
warnings by initializing these attributes without condition.

Added Bazel genrule for generating well_known_types_embed.cc

In pull request #2517 I made this change for the CMake and autotools
builds but forgot to do it for the Bazel build.

Add rvalue setters for non-arena strings on C++11.

Remove spurious NULL checks in ArenaStringPtr::CreateInstance.

Auto-generate well_known_types_embed.cc

The check target did not depend on the test_plugin binary, so this would
cause "make check" to fail at times because of the absence of
test_plugin. This change adds a dependency on test_plugin so that it
will always get built before the tests are executed.

Until now this file was just checked into the repo, but actually it
should be generated from any.js, struct.js, and timestamp.js. This
change updates the build system to make this happen. To make it work I
also had to remove some C++11 features from embed.cc.

Missed LIBPROTOC_EXPORT for GRPC added

Fixes #2511.

Define LANG_CXX11 for port.h and use this to guard C++11 features.

Removed Android-specific code from stubs/hash.h

Fixes and expands comments on how to use GPB_ENUM_FWD_DECLARE

This is basic and more tests will be added over time.

Fix integer overflow in FastUInt32ToBufferLeft

Add missing includes

Add support for Windows ARM builds