Projects which use Cap'n Proto require C++11, because they must be able to
compile our headers. Old gcc defaults to C++98, forcing users to manually
specify the -std=gnu++11 flag in their CMake projects. CMake 3.1
introduced the target_compile_features command which removes this
necessity by automatically communicating the C++ standard level
requirement to client projects.

Specifically, if target `kj` requires a C++11 feature publicly, then all
targets which link to `kj` will also require that C++11 feature, and get
the -std=gnu++11 flag for free. If that target is a library, such as
`kj-async`, and `kj-async` links publicly to `kj`, the requirement is also
transitive to all targets which link to `kj-async`.

Note that CMake's default behavior is to request compiler-specific
extensions, such as those provided by the -std=gnu++11 flag. You must
specifically opt out of these extensions. I'm not aware of any way to
propagate this nuance in CMake, but added a check to at least prevent
users from compiling Cap'n Proto without extensions.

With this change, the c++/samples/CMakeLists.txt will build with MinGW
with no change beyond requiring a later version of CMake.

This option was only useful when MSVC couldn't build the capnp command
line tools, so let's back it out.

Disallow MSVC from building the parsing code by default because the latest
stable MSVC (VS2015 Update 3) ICEs when trying to compile the `anyOfChars`
combinator.

Note that VS2015 Update 2 and VS2017 can compile the parsing code, with a
little help. To experiment with this, you can override the default
behavior with -DCAPNP_BUILD_TOOLS=ON.

Add missing include when building on Linux

Discovered trying to build on Android using CMake.  UIO_MAXIOV is only
exported by the linux/uio.h header which was never included.  Not sure
how normal Linux builds work.

Add missing reinterpret cast from const void* to const byte*.

Remove -pthread compile flag when building under Android.  ANDROID cmake
variable exported by android-cmake project.

autotools build does not have this option.
Capnp tools are built when CAPNP_LITE == OFF

Required by construct: export(EXPORT ..)

Build location is consistent with source and install location.

File CapnProtoConfig.cmake in build directory includes this file.
This change enables consumers to find package CapnProto
in build directory without installation.

Which tools should the build system use in this case?

This fixes the lib vs lib64 vs lib32 issue when installing

Other problem with cached BIN_INSTALL_DIR, LIB_INSTALL_DIR, etc options
is that changing official CMAKE_INSTALL_PREFIX variable has no effect.

This makes it a lot easier for CMake based projects to use Cap'n Proto.

Example usage:

find_package(CapnProto)
capnp_generate_cpp(FOO_SRCS FOO_HDRS foo.capnp)
add_executable(foo main.cpp ${FOO_SRCS})
target_link_libraries(foo CapnProto::capnp CapnProto::capnp-rpc)

This is a lot better than the previous variable based solution since
linking to nonexistent targets is an error whereas an empty variable
expansion (e.g. due to typos) will be silently ignored. It also makes
sure that the right compiler flags, include directories, defines and
link libraries are passed to the compiler for that target without
needing any other include_directories(), etc.

Otherwise the installed pkgconfig files are broken. The release script
has also been changed to update the version for CMakeLists.txt.

Fixes issue #162

I've added -Wextra as well as removed some of the -Wno-* flags and fixed issues in the code.

Also fixed the cmake build to put user-defined flags after default flags so that they can be overridden.

The idea behind the directory organization was that we might have official implementations for other languages in other top-level directories, as siblings to the c++ directory. It seems implausible that we'd use CMake as a meta-build-system over all of these.

Another reason for this change is that the release tarball actually contains only the c++ subdirectory. We probably want to include cmake files in the release.

CMake automatically patches the switch character.

With CAPNP_LITE=1, the command-line tools will not be built, and the
tests cannot be built without EXTERNAL_CAPNP=1 (BUILD_TESTING=0 needs to
be set if the tests are not desired).

* Move binaries into the bin directory and libs into the lib directory in the output
* Make sure the actual binary output file of capnp-tool is just capnp
* Remove need for deprecated cmake get_property function