CMake cleanup. (#384)

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.

.github/workflows/main.yml

@@ -18,12 +18,13 @@ jobs:
         # Build each combination of OS and release/debug variants
         os: [ "ubuntu-latest", "ubuntu-18.04", "macos-11", "macos-10.15", "freebsd-12.2", "freebsd-13.0" ]
         build-type: [ Release, Debug ]
-        extra-cmake-flags: [ " " ]
+        extra-cmake-flags: [ "" ]
         # Modify the complete matrix
         include:
           # Provide the dependency installation for each platform
           - os: "ubuntu-18.04"
             dependencies: "sudo apt install ninja-build"
+            cmake-flags: "-DSNMALLOC_USE_CXX17=ON"
           - os: "ubuntu-latest"
             dependencies: "sudo apt install ninja-build"
           - os: "macos-11"
@@ -68,7 +69,7 @@ jobs:
     - name: Install build dependencies
       run: ${{ matrix.dependencies }}
     - name: Configure CMake
-      run: cmake -B ${{github.workspace}}/build -DCMAKE_BUILD_TYPE=${{matrix.build-type}} -G Ninja ${{ matrix.extra-cmake-flags }}
+      run: cmake -B ${{github.workspace}}/build -DCMAKE_BUILD_TYPE=${{matrix.build-type}} -G Ninja ${{ matrix.cmake-flags }} ${{ matrix.extra-cmake-flags }}
       # Build with a nice ninja status line
     - name: Build
       working-directory: ${{github.workspace}}/build
@@ -201,7 +202,7 @@ jobs:
     steps:
     - uses: actions/checkout@v2
     - name: Configure CMake
-      run: cmake -B ${{github.workspace}}/build
+      run: cmake -B ${{github.workspace}}/build -DSNMALLOC_USE_CXX17=ON
     - name: Install clang-tidy
       run: sudo apt install clang-tidy-9
     # Run the clang-format check and error if it generates a diff
@@ -213,7 +214,7 @@ jobs:
         git diff --exit-code
     - name: Run clang-tidy
       run: |
-        clang-tidy-9  src/override/malloc.cc  -header-filter="`pwd`/*" -warnings-as-errors='*' -export-fixes=tidy.fail -- -std=c++17 -mcx16 -DSNMALLOC_PLATFORM_HAS_GETENTROPY=0
+        clang-tidy-9  src/override/malloc.cc  -header-filter="`pwd`/*" -warnings-as-errors='*' -export-fixes=tidy.fail -- -std=c++17 -mcx16 -DSNMALLOC_PLATFORM_HAS_GETENTROPY=0 
         if [ -f tidy.fail ] ; then
           cat tidy.fail
           exit 1