Add CMake project, GCC support (#540)

This is extract from #211 that enables compilation with GCC. With #211
now in the state of bitrot, I would rather try approaching it in smaller
steps that can be hopefully merged quicker, even if it does not provide
full support for all the features #211 provided.

This will _compile_ correctly with my (@janisozaur) GCC, but clang is
more picky about flexible array members and refuses to compile it yet.
I will extract remaining parts of #211 in future PRs.

I marked @fwcd as author, as he did most of the work in #211.

src/CMakeLists.txt

@@ -0,0 +1,11 @@
+cmake_minimum_required(VERSION 3.13)
+project(calculator CXX)
+set(CMAKE_CXX_STANDARD 17)
+
+if(CMAKE_CXX_COMPILER MATCHES ".*clang")
+    # Clang disagress with libstdc++ about constexpr-ness of wstring_view
+    # See https://github.com/Microsoft/calculator/pull/321
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -stdlib=libc++")
+endif()
+
+add_subdirectory(CalcManager)