Microsoft only implements putenv, which has a slightly different syntax.

This change allows CMake to build the entire project on Windows with MinGW
4.8.5.

Build samples with MSVC

Or... negato?

When building Cap'n Proto with compilers which can't yet build the command
line tools (MSVC), the exported targets capnp_tool and capnpc_cpp don't
exist, thus attempting to use them in a generator expression when setting
CAPNP_EXECUTABLE and CAPNPC_CXX_EXECUTABLE causes a hard error.

Leaving the variables unset provides users the opportunity to set
CAPNP_EXECUTABLE and CAPNPC_CXX_EXECUTABLE manually in CMake projects
consuming Cap'n Proto as a CMake package, and produces a less mystifying
error message if the user forgets.

Thanks to Philip Quinn for advice on this issue.

Fixes a recently introduced build error on Windows.

Microsoft's docs say to include either <process.h> or <stdlib.h>. I
arbitrarily chose process.h.

Refactor bounds checks to avoid ever creating out-of-bounds pointer values, which is technically UB even if not dereferenced.

Details: https://github.com/sandstorm-io/capnproto/blob/master/security-advisories/2017-04-17-0-apple-32bit-elides-bounds-check.md

MallocMessageBuilder: don't allocate segments above the max serializable size

Fix autotools build on MacOS

This eliminates a TODO(soon).

This allows functions which use KJ_EXPECT and other test macros to be built without linking libkj-test, which pulls in a possibly-unwanted main() function.

In particular, afl-testcase.c++ could not link in -fno-exceptions mode without this change.

doc/cxxrpc: Document lazy execution

The previous approach worked only for ekam builds, where fuzz-test was its own binary.