More CMake Improvements

Also, use pkg-config (if available) to get hints about the paths.

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).

Fix occasional evolution-test failure caused by List(Bool)->List(struct) upgrades no longer being allowed.

Fix `CAPNP_LITE` build

The changes to 'capnp/generated-header-support.h' in [b469079d] need to
be applied to both sides of the `#if CAPNP_LITE` conditional.

Conflicts:
	c++/src/capnp/generated-header-support.h

Fix install/uninstall automake hooks for CAPNP_LITE builds

Remove redundant 'const' qualifiers

Extended discussion:
https://groups.google.com/d/msg/capnproto/lRlWBOglQv4/8-Qo96AcZQIJ

The command-line tools aren't built in LITE_MODE, so there's nothing
to create a symlink to.

Extended discussion:
https://groups.google.com/d/msg/capnproto/lRlWBOglQv4/8-Qo96AcZQIJ

To use, pass --disable-reflection to the configure script.

This produces a smaller runtime library. However, using it for this purpose is not recommended. The main purpose of lite mode is to define a subset of Cap'n Proto which might plausibly compile under MSVC. MSVC still lacks full support for constexpr and expression SFINAE; luckily, most of our use of these things relates to reflection, and not all users need reflection.

Cap'n Proto lite mode inherits its name from Protocol Buffers' lite mode. However, there are some key differences:

- Protobuf generated code included global constructors related to registering descriptors and extensions. For many people, this was the main reason to use lite mode: to get rid of these global constructors and achieve faster startup times. Cap'n Proto, on the other hand, never had global constructors in the first place.

- Schemas are actually still available in lite mode, though only in their raw (Cap'n Proto structure) form. Only the schema API (which wraps the raw schemas in a more convenient interface) and reflection API (which offers a convenient way to use the schemas) are unavailable.

- Lite mode is enabled in an application by defining CAPNP_LITE rather than by specifying an annotation in the schema file. This better-reflects real-world usage patterns, where you typically want to enable lite mode application-wide anyway.

- We do not build the lite mode library by default. You must request it by passing --disable-reflection to the configure script. Before you can do that, you must have a prebuilt Cap'n Proto compiler binary available, since the compiler can't be built without reflection.

- Relatedly, the lite mode library is built with the same name as the full library. This library is not intended to be installed. If anything it should be statically linked. But, mostly the option only exists on non-MSVC platform to give us a way to test that we haven't broken lite mode.

Result -> Results

See changes to test.capnp for example.

Currently only supported through generated code, not in the dynamic API. Not exactly sure how to expose this in the dynamic API...

fix copy-paste bug in TestRpc_TailCall: add missing assertion

Note that embedded schema structures in generated code are still incorrect.

CMake improvements

initial fixes for building kj/main.c++ utilities on mingw

Remove '-d' flag from call to atos(1)

That is to say, whereas previously we would extend capnp::typeId<T>() to a new type by declaring a specialization of it for that type, now we instead have the type contain a nested class called _capnpPrivate which contains a `typeId` constant.

This is necessary because it is impossible to specialize a template for a type which is itself nested inside a template type. E.g. it's impossible to write a specialization `template <typename T> typeId<Foo<T>::Bar>()`; C++ simply doesn't support this. But with generics, Cap'n Proto will allow types to be nested inside templates, so we need this to work.