Switched QHull ImGUI and GoogleTest to use FetchContent (#6645)

* Switched QHull and GoogleTest to use FetchContent to populate their files at configure time. This means that the open3d_build_3rdparty_library function doesn't need to mark source files as generated which was causing them to be removed when running the clean target.
* Switched ImGUI to use FetchContent instead of external project so files are provided at configure time instead of build time.
* Removed vestigial ext_imgui target that used to download the imgui sources. Sources are now downloaded at configure time.

3rdparty/find_dependencies.cmake

@@ -96,8 +96,6 @@ function(open3d_build_3rdparty_library name)
     if(arg_SOURCES)
         foreach(src IN LISTS arg_SOURCES)
             get_filename_component(abs_src "${src}" ABSOLUTE BASE_DIR "${arg_DIRECTORY}")
-            # Mark as generated to skip CMake's file existence checks
-            set_source_files_properties(${abs_src} PROPERTIES GENERATED TRUE)
             target_sources(${name} PRIVATE ${abs_src})
         endforeach()
         foreach(incl IN LISTS include_dirs)
@@ -1030,8 +1028,6 @@ if(NOT USE_SYSTEM_QHULLCPP)
             src/libqhull_r/rboxlib_r.c
         INCLUDE_DIRS
             src/
-        DEPENDS
-            ext_qhull
     )
     open3d_build_3rdparty_library(3rdparty_qhullcpp DIRECTORY ${QHULL_SOURCE_DIR}
         SOURCES
@@ -1057,8 +1053,6 @@ if(NOT USE_SYSTEM_QHULLCPP)
             src/libqhullcpp/RoadLogEvent.cpp
         INCLUDE_DIRS
             src/
-        DEPENDS
-            ext_qhull
     )
     target_link_libraries(3rdparty_qhullcpp PRIVATE 3rdparty_qhull_r)
     list(APPEND Open3D_3RDPARTY_PRIVATE_TARGETS_FROM_CUSTOM Open3D::3rdparty_qhullcpp)
@@ -1158,8 +1152,6 @@ if (BUILD_UNIT_TESTS)
                 googletest/
                 googlemock/include/
                 googlemock/
-            DEPENDS
-                ext_googletest
         )
     endif()
 endif()
@@ -1190,8 +1182,6 @@ if(BUILD_GUI)
                 imgui_tables.cpp
                 imgui_widgets.cpp
                 imgui.cpp
-            DEPENDS
-                ext_imgui
         )
         list(APPEND Open3D_3RDPARTY_PRIVATE_TARGETS_FROM_CUSTOM Open3D::3rdparty_imgui)
     else()

3rdparty/googletest/googletest.cmake

@@ -1,6 +1,6 @@
-include(ExternalProject)
+include(FetchContent)
 
-ExternalProject_Add(
+FetchContent_Declare(
     ext_googletest
     PREFIX googletest
     URL https://github.com/google/googletest/archive/refs/tags/release-1.11.0.tar.gz
@@ -12,5 +12,5 @@ ExternalProject_Add(
     INSTALL_COMMAND ""
 )
 
-ExternalProject_Get_Property(ext_googletest SOURCE_DIR)
-set(GOOGLETEST_SOURCE_DIR ${SOURCE_DIR})
+FetchContent_Populate(ext_googletest)
+FetchContent_GetProperties(ext_googletest SOURCE_DIR GOOGLETEST_SOURCE_DIR)

3rdparty/imgui/imgui.cmake

@@ -1,6 +1,6 @@
-include(ExternalProject)
+include(FetchContent)
 
-ExternalProject_Add(
+FetchContent_Declare(
     ext_imgui
     PREFIX imgui
     URL https://github.com/ocornut/imgui/archive/refs/tags/v1.88.tar.gz
@@ -12,5 +12,5 @@ ExternalProject_Add(
     INSTALL_COMMAND ""
 )
 
-ExternalProject_Get_Property(ext_imgui SOURCE_DIR)
-set(IMGUI_SOURCE_DIR ${SOURCE_DIR})
+FetchContent_Populate(ext_imgui)
+FetchContent_GetProperties(ext_imgui SOURCE_DIR IMGUI_SOURCE_DIR)

3rdparty/qhull/qhull.cmake

@@ -1,6 +1,6 @@
-include(ExternalProject)
+include(FetchContent)
 
-ExternalProject_Add(
+FetchContent_Declare(
     ext_qhull
     PREFIX qhull
     # v8.0.0+ causes seg fault
@@ -14,5 +14,5 @@ ExternalProject_Add(
     INSTALL_COMMAND ""
 )
 
-ExternalProject_Get_Property(ext_qhull SOURCE_DIR)
-set(QHULL_SOURCE_DIR ${SOURCE_DIR})
+FetchContent_Populate(ext_qhull)
+FetchContent_GetProperties(ext_qhull SOURCE_DIR QHULL_SOURCE_DIR)