Shameless plug, but check out the Sokol libraries (there's also a header with a Dear ImGui rendering backend sitting on top of sokol_app.h and sokol_gfx.h):
...and here is a slightly more rich starter project that happens to use Sokol + Dear ImGui. It was made to create game tools fast in Zig. https://github.com/prime31/zig-upaya
https://github.com/floooh/sokol
...and here's a minimal standalone starter project for writing Dear ImGui apps in C I just created a couple of days ago:
https://github.com/floooh/cimgui-sokol-starterkit