Fixes deprecation warnings/errors on Xcode

Added explicit cast to avoid size warning on Win64.

Use latest maven-compiler-plugin (2.6.0)

Renamed the pnacl version of atomicops.h into C11 atomic, and flagged the mac version to that if atomic is enabled

Fix unused parameter warnings in arena_free

Fix message for InvalidProtocolBufferException

The size parameter is only used in cases where the compiler supports
sized delete, so when that's not available we need to specifically cast
it to void to prevent a warning.

* Uses build-helper-maven-plugin to add generated sources to the classpath
* Fixes an issue building with newer versions of the maven-compiler-plugin
  (See https://issues.apache.org/jira/browse/MCOMPILER-240)

[arm/gcc] Don't rely on KUSER_HELPERS feature for atomics

This affects cases with leading capital letters.

This breaks compatibility with previous C# releases, but
fixes compatibility with other implementations.

See #2278 for details.

generic atomicops: Use strong compare_exchange

Fix compiler warnings when running :protobuf_test

When running >bazel test :protobuf_test a few compiler warnings came up.  They were centered around the usage of "int" as the loop counter where the value should have been a size_t.

Docs: Link to Apple's Swift plugin and runtime for protobufs

Add note about extension use and Coding support.

Weak compare-and-exchange are allowed to fail spuriously, so we have
to use the strong variation here.

ARM specific CONFIG_KUSER_HELPERS feature can be disabled in Linux kernel,
and in this case, we shouldn't crash. Use gcc built-in functions instead
of arm specific code if they are available.

Adds pushLimit: and popLimit: into GPBCodedInputStream

define no_threadlocal on OpenBSD

Support extra parameters for plugins.

Hacking ByteBufferWriter to work with GAE

Fixes #2269

Place Python extensions correctly in Bazel build.

Use git clean before installing via bundler