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/CalcViewModel/UnitConverterViewModel.cpp

@@ -675,7 +675,7 @@ void UnitConverterViewModel::RefreshCurrencyRatios()
     String ^ announcement = AppResourceProvider::GetInstance().GetResourceString(UnitConverterResourceKeys::UpdatingCurrencyRates);
     Announcement = CalculatorAnnouncement::GetUpdateCurrencyRatesAnnouncement(announcement);
 
-    auto refreshTask = create_task(m_model->RefreshCurrencyRatios());
+    auto refreshTask = create_task([this] { return m_model->RefreshCurrencyRatios().get(); });
     refreshTask.then(
         [this](const pair<bool, wstring>& refreshResult) {
             bool didLoad = refreshResult.first;