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Remove -DCMAKE_BUILD_WITH_CMAKE from Source/CMakeLists.txt. Remove
CMAKE_BOOTSTRAP from bootstrap's cmConfigure.h. Add -DCMAKE_BOOTSTRAP
to all bootstrap sources, and remove the Unix specialization of
CMAKE_BOOTSTRAP in libuv (a review of the libuv code suggests that
this will not have an effect on the Windows build.)

In commit a605bf43 (Extend C++17/C++14 feature checks to cover more
standard library APIs, 2019-02-27, cpp-modules-20190312.1~52^2~1) we
added checks to the main build of CMake to verify C++14 capabilities
before using C++14 flags to build.  Add these to the bootstrap check
too.

Co-Authored-by: Bryon Bean <bryon.bean@kitware.com>

This adds a new RAII based cmFileTimes class.
It is supposed to replace the C style cmSystemToolsFileTime interface.

Previously CMake used fairly naive logic to determine when to do
device linking which caused unnecessary device linking to occur
frequently. We now use a more exhaustive algorithm to determine
when we have a need for device linking.

Fixes: #19238

This class is ultimately intended as a replacement for cmsys::Process.
It spawns a series of processes using libuv, piping the output of each
command into the next.

Note: input support has not yet been implemented because write
support has not yet been implemented on cmUVStreambuf.

The name `cmFileTimeCache` reflects the functionality of the class more
appropriately.

The new cmFileTime class stores the file modification time as an OS independent
nanosecond count.  Its main use is to load and compare file modification times
in nanosecond and second resolution.

Split these classes out into their own sources.

Build zstd as part of CMake or find one on the system.  Modify our
port of libarchive to use the zstd configured for use with CMake.

Issue: #18657

In order to keep infinitely-recursive scripts from causing a stack
overflow in the CMake executable, CMake now imposes a maximum
recursion limit before issuing an error message. The limit can be
adjusted at runtime with CMAKE_MAXIMUM_RECURSION_DEPTH.

Fixes: #18694

Define a `cm::string_view` type implemented via C++17 `std::string_view`
when available.  Provide a fallback implementation for C++11 and C++14
compilers.

The fallback implementation was written by reading documentation of the
standard spec.  We have no dedicated tests for it, but it will be
covered by tests of its clients later.

Before this change, install rules created by add_subdirectory()
would be executed after all of the top-level install rules, even
if they were declared before the top-level rules. This change
adds a new policy, CMP0082, which interleaves the add_subdirectory()
install rules with the other install rules so they are run in the
correct order.

Closes #18375

Avoid exposing the item name implicitly as std::string.  When the item
is a target, avoid storing a second copy of its name.

Most link item construction is paired with calls to `FindTargetToLink`
to get the possible target pointer.  Rename these methods to
`ResolveLinkItem` and refactor them to construct the entire item.

Add option `SPHINX_INFO` to enable the Sphinx 'texinfo' builder and use
the `makeinfo` tool to convert it to a `.info` file.

Mention the flags used when complaining that the compiler does not work.

Add `clang` and `clang++` after the corresponding `gcc` and `g++`
candidates.  Otherwise it is not tried on platforms where we do not
enumerate the toolchain pair candidates.

Some versions of clang 5 (with libc++) have a problem with
`unordered_map` under `-std=gnu++1z`:

    /usr/include/c++/__hash_table:1134:43: error: conflicting types for '__hash_table<_Tp, _Hash, _Equal, _Alloc>'

Include `unordered_map` in our test source so that we reject this
combination and fall back to an older C++ standard flag.

Fixes: #17526

In our loop to detect -std flags, add another layer to detect
threading flags.