[build] Fix Findimgui.cmake -- still not working since libimgui.so do…

…esn't include backends defs

(cherry picked from commit d12b7aa0e1c2738fbfe817f59bb0424d4161bd02)

cmake/Findimgui.cmake

@@ -19,13 +19,33 @@ find_package(PkgConfig QUIET)
 # Check to see if pkgconfig is installed.
 pkg_check_modules(PC_imgui imgui QUIET)
 
-# Include directories
-find_path(imgui_INCLUDE_DIRS
+# Find the path containing imgui.h
+find_path(imgui_INCLUDE_DIR
   NAMES imgui.h
   HINTS ${PC_imgui_INCLUDEDIR}
-  PATHS "${CMAKE_INSTALL_PREFIX}/include"
+  PATHS
+    "${CMAKE_INSTALL_PREFIX}/include"
+    "${CMAKE_INSTALL_PREFIX}/include/imgui"
 )
 
+# Find the path containing imgui_impl_opengl2.h
+find_path(imgui_backends_INCLUDE_DIR
+  NAMES imgui_impl_opengl2.h
+  HINTS
+    ${PC_imgui_INCLUDEDIR}
+    ${PC_imgui_INCLUDEDIR}/backends
+  PATHS
+    "${CMAKE_INSTALL_PREFIX}/include"
+    "${CMAKE_INSTALL_PREFIX}/include/imgui/backends"
+)
+
+# Combine both paths into imgui_INCLUDE_DIRS
+if (imgui_INCLUDE_DIR AND imgui_backends_INCLUDE_DIR)
+  set(imgui_INCLUDE_DIRS ${imgui_INCLUDE_DIR} ${imgui_backends_INCLUDE_DIR})
+else()
+  message(FATAL_ERROR "Could not find the required imgui headers")
+endif()
+
 # Library
 find_library(imgui_LIBRARIES
   NAMES imgui