Add build2 project config variables for imconfig.h

[kamrann]

I needed to use ImGui with 32 bit indices enabled. I've added variables for a number of other ImGui configuration options along with it, but not all.

• There are some options related to STB/truetype/freetype; I've not touched these as the comments mention backends and it's not clear to me if this implies they're implemented somewhere else, in which case maybe there is no equivalent build2 package/the option should be provided elsewhere.
• IMGUI_INCLUDE_IMGUI_USER_H doesn't seem useful in the context of a build2 package.
• Likewise ImDrawCallback mentions a need to modify backends accordingly so can't be meaningfully configured on its own.
• There are a few others that just seem very specific and I'm unsure if it would be worth trying to reflect everything.

Resolves #3

[Klaim]

I think it's a decent start for now. Could you add a list of supported config options in the Readme, with a link to the config header upstream to refer to for documentation? That way we wouldnt have to do much other than check if something new needs to be added as config option.

[boris-kolpackov]

@Rookfighter Will you be able to take a look at this PR some time soon? Thanks!