Clarify RPC spec documentation based on mail thread

Kenton rightly pointed out that this is an unnecessary round-trip.

Update README.md

protobuf link update

Currently only implemented for Linux (when using futexes), since that's what I use. Will expand to other platforms later.

synchrozines behavior with autoconf build

https://github.com/capnproto/capnproto/issues/515

https://groups.google.com/d/topic/capnproto/_1kBtRSC51s/discussion

Doc: Enforce comment-after-code rule

Install capnp-json.pc

While comments should appear after the code, some examples violate the convention. This commit enforces the rule.

Minor: fix typo in `capnp help encode` documentation

Add versioning to the libraries in the CMake build

The Autotools and CMake builds are almost the same, except for
the libraries not being generated as versioned libraries in CMake.

Fixing this lets Linux CMake builds be on the same level as Autotools ones.

Add cmake_minimum_required to root CMakeLists.txt

There was already a minimum version specified in the actual CMakeLists
file in the C++ directory, but since the parent file does not have it
you get a warning if you build in the idiomatic cmake fashion:

    mkdir build
    cd build
    cmake .. # <---- warning here because no minimum version specified

I used 3.1 as min version because that is the same as specified in child
file.

This reverts commit b50f6598.

Merge branch 'doccomments' of https://github.com/opensourcerouting/capnproto into opensourcerouting-doccomments

Fix MSVC build error when using List<T> with generic T

Add CMakeLists.txt to top level

Tweaks and bugfixes

It turns out that, quite often, the write() will have already completed before the method returns, hence dropping the returned promise won't cause any apparent problem -- until you try to write() a sufficiently large value that it doesn't complete on the first call. Then, you get mysterious bugs.

Indeed, it turns out the HTTP library has such bugs, which are fixed in this change.