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/CalculatorUnitTests/UnitConverterViewModelUnitTests.h

@@ -47,7 +47,7 @@ namespace CalculatorUnitTests
         void ResetCategoriesAndRatios() override
         {
         }
-        concurrency::task<std::pair<bool, std::wstring>> RefreshCurrencyRatios() override
+        std::future<std::pair<bool, std::wstring>> RefreshCurrencyRatios() override
         {
             co_return std::make_pair(true, L"");
         }