CMake 3.12 deprecated `FindPythonInterp`, and with CMake 3.27, were
obsoleted with CMP0148.  Add a version check and switch to the new
behaviour to allow building with newer releases.  When the minimum CMake
version is increased, we can look at dropping the compatibility path.

Adjust the `add_test` invocation to clearly indicate the test name and
command parameters.

`BUILD_TESTING` is the CMake sanctioned option for the control of tests
(c.f. https://cmake.org/cmake/help/latest/module/CTest.html).  Use the
standard option to control the build of test targets.

Policy settings may impact how `project` functions.  They should be set
immediately after `cmake_minimum_required` (which implicitly sets
policies).

Use the `POLICY` check to see if a policy is defined rather than using a
version check.

This variable was no longer in use and the recent cleanups had exposed
it as being unnecessary.

The `CMARK_STATIC_DEFINE` macro identifies that the library is being
linked statically, which is important information for building on
Windows. This uses CMake to automatically propagate the necessary
definition to the users and the build of the library itself rather than
having everyone specify it manually.

Use generator expressions to compute the path rather than hardcoding the
layout.  Finally, clean up some unnecessary quoting and uniformly spell
commands in the tests.

Match the style as recommended by CMake documentation in [1]. This makes
it easier to read the command being invoked when running tests.  No
functional change.

[1] https://cmake.org/cmake/help/latest/command/add_test.html

CMake 3.15+ remove `/W3` from the language flags under MSVC with
CMP0092.  Set the policy to new to avoid the D9025 warning.

`stdbool.h` is part of C99 however was not provided by Visual Studio
2013 until RTM [1]. Remove the check for the header and inline the
include at the usage sites rather than relying on `config.h`.  VS2013
was EOL'ed Apr 9, 2019, with extended support ending Apr 9, 2024.

`HAVE___ATTRIBUTE__` was unused in the codebase and served no purpose.

Remove shims for `snprintf` and `vsnprintf` which were unavailable prior
to VS2015.  As VS2013 is no longer serviced, this reduces complexity in
the project.

Avoid including the utility once, which should avoid some unnecessary
CMake checks, and reduces duplication.

man pages are extremely useful, but are not generally available on
Windows.  This changes the install condition to check for the Windows
cross-compile rather than the toolchain in use.  It is possible to build
for Windows using clang in the GNU driver.

* build: inline a variable

* build: use `LINKER_LANGUAGE` property for C++ runtime

Rather than explicitly name the C++ runtime, use the `LINKER_LANGUAGE`
property to use the driver to spell the C++ runtime appropriately.

* build: use CMake to control C standard

Rather than use compiler specific flags to control the language
standard, indicate to CMake the desired standard.

* build: use the correct variable

These flags are being applied to the *C* compiler, check the C compiler,
not the C++ compiler.

* build: loosen the compiler check

This loosens the compiler identifier check to enable matching AppleClang
which is the identifier for the Xcode compiler.

* build: hoist shared flags to top-level CMakeLists

This hoists the common shared flags handling to the top-level CMakeLists
from sub-layers.  This prevents the duplication of the handling.

* build: remove duplicated flags

This is unnecessary, `/TP` is forced on all MSVC builds, no need to
duplicate the flag for older versions.

* build: loosen C compiler identifier check

Loosen the check to a match rather than equality check, this allows it
to match AppleClang which is the identifier for the Apple vended clang
compiler part of Xcode.

* build: use `add_compile_options`

Use `add_compile_options` rather than modify `CMAKE_C_FLAGS`.  The
latter is meant to be only modified by the user, not the package
developer.

* build: hoist sanitizer flags to global state

This moves the CMAKE_C_FLAGS handling to the top-level and uses
`add_compile_options` rather than modifying the user controlled flags.

* build: hoist `-fvisibilty` flags to top-level

These are global settings, hoist them to the top level.

* build: hoist the debug flag handling

Use a generator expression and hoist the flag handling for the debug
build.

* build: hoist the profile flag handling

This is a global flag, hoist it to the top level and use
`add_compile_options` rather than modify the user controlled flags.

* build: remove incorrect variable handling

This seemed to be attempting to set the linker not the linker flags for
the profile configuration.  This variable is not used, do not set it.

* build: remove unused CMake includes

Newer versions of CMake attempt to query the system for information about the VS
2017 installation.  Unfortunately, this query fails on non-Windows systems when
cross-compiling:

	cmake_host_system_information does not recognize <key> VS_15_DIR

CMake will not find these system libraries on non-Windows hosts anyways, and we
were silencing the warnings, so simply omit the installation when
cross-compiling to Windows.

Remove the use of `CMARK_INLINE` (as is being considered upstream) as
this workaround is no longer needed. It was to support VS2013 which has
been obsoleted for some time now.

Restructure the CMake modules to match expectations for CMake.

Avoid defining `likely` and `unlikely` in global scope. While doing so,
prefer the modern `__has_builtin` for the check (with fallbacks for
pre-GCC 4.10, clang 3.4, and MSVC). This largely shouldn't matter in
practice as modern branch predictors do well without the hint. Sink the
declaration into source files as the macros are not namespaced.

Remove the now unnecessary `config.h`.

Clean up the builds to only support a single build type which is
dependent on the standard `BUILD_SHARED_LIBS`.

Simplify and normalise the install rules to use the `GNUInstallDirs`
parameters.