diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 64676004..3aff16f7 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.16...3.22)
+cmake_minimum_required(VERSION 3.16...3.23)
 
 # set a default CXX standard used by the external tools like clang-tidy, cppcheck, etc.
 # You can later set fine-grained standards for each target using `target_compile_features`
@@ -38,7 +38,7 @@ project_options(
   # TODO(CK: should not installed with this option!) ENABLE_COVERAGE
   # TBD: ENABLE_PCH
   # TBD: PCH_HEADERS ${PCH_HEADERS}
-  ENABLE_DOXYGEN
+  # NO(CK)! To slow! ENABLE_DOXYGEN
   # ENABLE_INTERPROCEDURAL_OPTIMIZATION
   # ENABLE_NATIVE_OPTIMIZATION
   # ENABLE_USER_LINKER
@@ -74,9 +74,10 @@ target_link_system_libraries(
 enable_testing()
 add_test(NAME main COMMAND main)
 
-### to get CMAKE_INSTALL_INCLUDEDIR
-# NOTE: enable at least one language before including GNUInstallDirs.
-include(GNUInstallDirs)
+if(NOT DEFINED CMAKE_INSTALL_INCLUDEDIR)
+  # NOTE: enable at least one language before including GNUInstallDirs.
+  include(GNUInstallDirs)
+endif()
 
 # Header-only library
 add_library(lib INTERFACE)
diff --git a/test_install/CMakeLists.txt b/test_install/CMakeLists.txt
index 0d4220e6..40bf9ec0 100644
--- a/test_install/CMakeLists.txt
+++ b/test_install/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.16...3.22)
+cmake_minimum_required(VERSION 3.16...3.23)
 
 set(CMAKE_CXX_STANDARD 20)
 
@@ -23,11 +23,11 @@ project_options(
   # ENABLE_COVERAGE
   # ENABLE_PCH
   # PCH_HEADERS <Eigen/Dense> <fmt/core.h> <vector> <utility> <string> <string_view>
-  ENABLE_DOXYGEN
+  # XXX ENABLE_DOXYGEN
   DOXYGEN_THEME
   "${CMAKE_CURRENT_LIST_DIR}/css/my_custom_theme.css"
   "${CMAKE_CURRENT_LIST_DIR}/css/my_custom_theme_extra.css"
-  ENABLE_INTERPROCEDURAL_OPTIMIZATION
+  # ENABLE_INTERPROCEDURAL_OPTIMIZATION
   # ENABLE_USER_LINKER
   # ENABLE_BUILD_WITH_TIME_TRACE
   # ENABLE_UNITY