https://github.com/ocornut/imguiHere is a rough guide how to get it running using SDL2 or 3.
* Common
imconfig.h:
To fix colors, add:
    #define IM_COL32_R_SHIFT    24
    #define IM_COL32_G_SHIFT    16
    #define IM_COL32_B_SHIFT    8
    #define IM_COL32_A_SHIFT    0
    #define IM_COL32_A_MASK     0x000000FF
Enable following to avoid code which uses fork():
    #define IMGUI_DISABLE_DEFAULT_SHELL_FUNCTIONS
* MiniGL (example_sdl2_opengl2)
main.cpp:
Modify GL version to supported:
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 1);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 3);
* OpenGLES 2 (example_sdl2_opengl3, example_sdl3_opengl3)
Makefile:
Add:
-DIMGUI_IMPL_OPENGL_ES2
Demo: 
https://capehill.kapsi.fi/imgui/imgui_amigaos4.lha (MiniGL or OpenGLES2 required).