--- a/ext/CMakeLists.orig.txt	2020-06-06 20:38:54.795020000 +0200
+++ b/ext/CMakeLists.txt	2020-06-06 20:41:11.429317416 +0200
@@ -1,63 +1,6 @@
 # Niels Lohmann's json library (single-header version).
 
-add_library(ext-json INTERFACE)
-target_include_directories(ext-json INTERFACE json/single_include)
-
 # tinyopt command line parsing libary (header-only).
 
 add_library(ext-tinyopt INTERFACE)
 target_include_directories(ext-tinyopt INTERFACE tinyopt/include)
-
-# Random123 (DE Shaw Research) counter-based random number generators (header-only)
-
-add_library(ext-random123 INTERFACE)
-target_include_directories(ext-random123 INTERFACE $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>/random123/include)
-install(TARGETS ext-random123 EXPORT arbor-targets)
-
-# Google benchmark for microbenchmarks:
-
-check_git_submodule(gbench google-benchmark)
-if(gbench_avail)
-    # Set up google benchmark as an external project.
-
-    include(ExternalProject)
-    set(gbench_src_dir "${CMAKE_CURRENT_SOURCE_DIR}/google-benchmark")
-    set(gbench_install_dir "${CMAKE_CURRENT_BINARY_DIR}/google-benchmark")
-    set(gbench_cmake_args
-        "-DCMAKE_BUILD_TYPE=release"
-        "-DCMAKE_INSTALL_PREFIX=${gbench_install_dir}"
-        "-DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}"
-        "-DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}")
-
-    ExternalProject_Add(gbench-build
-        # Add dummy DOWNLOAD_COMMAND to stop ExternalProject_Add terminating CMake if the
-        # git submodule had not been udpated.
-        DOWNLOAD_COMMAND "${CMAKE_COMMAND}" -E echo "Warning: ${gbench_src_dir} empty or missing."
-        SOURCE_DIR "${gbench_src_dir}"
-        CMAKE_ARGS "${gbench_cmake_args}"
-        INSTALL_DIR "${gbench_install_dir}"
-    )
-    set_target_properties(gbench-build PROPERTIES EXCLUDE_FROM_ALL TRUE)
-
-else()
-    add_error_target(gbench-build
-        "Building Google benchmark library"
-        "The git submodule for google benchmark is not available")
-endif()
-
-add_library(ext-benchmark INTERFACE)
-add_dependencies(ext-benchmark gbench-build)
-target_include_directories(ext-benchmark INTERFACE "${gbench_install_dir}/include")
-target_link_libraries(ext-benchmark INTERFACE "${gbench_install_dir}/lib/libbenchmark.a")
-
-# Sphinx RTD theme
-
-check_git_submodule(rtdtheme sphinx_rtd_theme)
-add_target_if(rtdtheme_avail
-    check-sphinx_rtd_theme
-    "Checking for Read the Docs Sphinx theme"
-    "The git submodule for read the docs is not available")
-
-add_library(ext-sphinx_rtd_theme INTERFACE)
-add_dependencies(ext-sphinx_rtd_theme check-sphinx_rtd_theme)
-target_include_directories(ext-sphinx_rtd_theme INTERFACE sphinx_rtd_theme)