Protobuf now uses these two features, which are not available on some
platforms.

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.

Make the code match what the docs claimed, to return a define for HAVE_UNORDERED_MAP

This patch adds support for atomic operations on Solaris, on any platform.
It makes use of the atomic functions made available in Solaris' atomic.h
header.

Patch from Monty Taylor <monty.taylor@gmail.com>.

committer: Jon Skeet <skeet@pobox.com>