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).
- Clean up the options and use dependent options to hide options
that are not always relevant.
- 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.
- Make the linker error if the shim libraries depend on symbols that
are not defined in the explicitly-provided libraries.
- Make the old-Ubuntu CI jobs use C++17 explicitly (previously CMake
was silently ignoring the fact that the compiler didn't support C++20)
- Fix errors found by the more aggressive linking mode.
With these changes, it's now possible to install snmalloc and then, in
another project, do something like this:
```cmake
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.
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