Failing to build because of imgui

[define-private-public]

I checked out this repo on a fairly vanilla system and tried to build on Ubuntu 18.04 using GCC 10.x. I installed the LunarG sdk, glm, etc. I tried building, but it's failing at the step where I tries to locale imgui:

CMake Error at cmake/macros/TargetImgui.cmake:2 (find_package):
  Could not find a package configuration file provided by "imgui" with any of
  the following names:

    imguiConfig.cmake
    imgui-config.cmake

  Add the installation prefix of "imgui" to CMAKE_PREFIX_PATH or set
  "imgui_DIR" to a directory containing one of the above files.  If "imgui"
  provides a separate development package or SDK, be sure it has been
  installed.
Call Stack (most recent call first):
  base/CMakeLists.txt:35 (target_imgui)

I tried cloning the Dear imGui repo and setting the imgui_DIR environment variable to point it it, and I got the same result.