audit C ABI compatibility for extern enums

[andrewrk]

• do some C environments let you specify the tag type?
• do some C environments make enums be c_long if you use a long const value or c_longlong if you use a long long const value?
• should specifying tag type of extern enums be a compile error?

What it looks like to solve this issue is investigate these questions for all our target C environments:

• gnu
• msvc
• cygwin
• mysys? (windows + gnu)

[isaachier]

This reminds me of a funny comment I came across recently:

// C++ is really really really really really really dumb about enums, so screw that and just make them anonymous
// This has the advantage of being ABI-able should we ever need an ABI...

https://github.com/andlabs/libui/blob/master/ui.h#L32-L34

[isaachier]

Relevant issue in Vulkan docs: KhronosGroup/Vulkan-Docs#124.