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/CalcManager/Header Files/EngineStrings.h

@@ -16,7 +16,10 @@
 
 #pragma once
 
+#include <array>
 #include <string>
+#include <string_view>
+#include <unordered_map>
 
 inline constexpr auto IDS_ERRORS_FIRST = 99;