CMake cleanup.

[David Chisnall (davidchisnall)]

Modernise and tidy the CMake a bit:

• Use generator expressions for a lot of conditionals so that things
  are more reliable with multi-config generators (and less verbose).
• Remove C as a needed language. None of the code was C but we were
  using C to test if headers worked. This was fragile because a build
  with CMAKE_CXX_COMPILER set might have checked things compiled with
  the system C compiler and then failed when the specified C++ compiler
  used different headers.
• Rename the BACKTRACE_HEADER macro to SNMALLOC_BACKTRACE_HEADER.
  This is exposed into code that consumes snmalloc and so should be
  'namespaced' (to the degree that's possible with C macros).
• Use functions instead of macros for better variable scoping.
• Factor out some duplicated bits into functions.
• Update to the latest way of telling CMake to use C++17 or C++20.
• Migrate everything that's setting global properties to setting only
  per-target properties.
• Link with -nostdlib++ if it's available. If it isn't, fall back to
  enabling the C language and linking with the C compiler.
• Make the per-test log messages verbose outputs. These kept scrolling
  important messages off the top of the screen for me.
• Make building as a header-only library a public option.
• Add install targets that install all of the headers and provide a
  config option. This works with the header-only configuration for
  integration with things like vcpkg.
• Fix a missing #endif in the malloc_useable_size check. This was
  failing co compile on all platforms because of the missing #endif.
• Bump the minimum version to 3.14 so that we have access to
  target_link_options. This is necessary to use generator expressions
  for linker flags.

With these changes, it's now possible to install snmalloc and then, in
another project, do something like this:

find_package(snmalloc CONFIG REQUIRED)
target_link_libraries(t1 snmalloc::snmalloc)
target_link_libraries(t2 snmalloc::snmallocshim-static)

In this example, t1 gets all of the compile flags necessary to include
snmalloc headers for its build configuration. t2 is additionally
linked to the snmalloc static shim library.