Improve cross platform compliance (#32)

* Add missing `Skyr_DISABLE_LIBCXX` option deceleration in top level CMakeLists.txt

The option deceleration is missing, yet it is used for setting up the
`stdlib=stdc++` flag for clang. Cmake option has the default value of `OFF`;
the value is negated in the if statement so by default the flag is appended
regardless of whether the machine has `stdc++` installed. Since some Clang
installations do not come with `stdc++`, the project cannot be built.

The default value is set to `ON` since LLVM's implementation of C++17 is not
yet complete. More can be found in the links below:
- [cppreference](https://en.cppreference.com/w/cpp/compiler_support)
- [libc++ on 17](https://libcxx.llvm.org/cxx1z_status.html)
- [libc++ homepage](https://libcxx.llvm.org/index.html)

* Improve standard compliance

By default if `CMAKE_CXX_EXTENSIONS` and `CMAKE_CXX_STANDARD_REQUIRED`
are not set, cmake would allow non-standard C++ extensions. For instance
it would pass `-std=gnu++17` instead of `-std=c++17` to GCC.

Author: Internal-Compiler-Error

CMakeLists.txt

@@ -15,9 +15,12 @@ option(Skyr_WARNINGS_AS_ERRORS "Treat warnings as errors." ON)
 option(Skyr_BUILD_WITHOUT_EXCEPTIONS "Build without exceptions." OFF)
 option(Skyr_USE_STATIC_CRT "Use static C Runtime library (/MT or MTd)." ON)
 option(Skyr_BUILD_FILESYSTEM_PATH_FUNCTIONS "Build the filesystem path functions" OFF)
+option(Skyr_DISABLE_LIBCXX "Instruct Clang to use LLVM's implementation of C++ standard library" ON)
 
 set(CMAKE_VERBOSE_MAKEFILE true)
 set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_EXTENSIONS OFF)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
 find_package(Threads REQUIRED)
 find_package(tl-expected CONFIG REQUIRED)