Make Crow compile on MacOS

[The-EDev]

Needs some testing (primarily creating a blueprint with a custom static directory)
I'm also not sure if this is the right way to do things, I mainly changed things until the code compiled.

fixes #229

[luca-schlecker]

Is there a way to reproduce #229 on Linux or Windows or is it a problem with the Apple C++ Standard Library?
Standard says that operator=(std::initializer_list<value_type> ilist) is defined for std::map and std::unordered_map with value_type = std::pair<const Key, T>.

Edit: Seems like there is no way. It looks like it's specifically a problem with the C++ standard library for Apple.

[The-EDev]

Is there a way to reproduce #229 on Linux or Windows or is it a problem with the Apple C++ Standard Library?

It seems to be related to Apple's standard library, since it's a compiler error that doesn't occur on Linux or Windows. We could add a macro specifically to check whether MacOS is being used and change the code based on that, I think that might be a better approach than creating new values.

[luca-schlecker]

I think including that if is sufficient as it doesn't really impact the rest.
If really wanted, that if could be wrapped inside a compiler macro disabling it for anything other than apple.
Something like this should work for detecting if on apple:

#if defined(__APPLE__) || defined(__MACH__)
...
#endif

[luca-schlecker]

LGTM, though I didn't test it, as I don't have an osx device. 🎉

[The-EDev]

Just tested on my MacOS machine, works like a charm. Merging now.