diff --git a/cmake/InstallRules.cmake b/cmake/InstallRules.cmake
index b245a22a5..b8c683c91 100644
--- a/cmake/InstallRules.cmake
+++ b/cmake/InstallRules.cmake
@@ -3,14 +3,14 @@ include(CMakePackageConfigHelpers)
 # copy header files to CMAKE_INSTALL_INCLUDEDIR
 install(
     DIRECTORY
-    "${PROJECT_SOURCE_DIR}/include"  # our header files
-    "${PROJECT_BINARY_DIR}/include"  # generated header files
+    "${PROJECT_SOURCE_DIR}/include/"  # our header files
+    "${PROJECT_BINARY_DIR}/include/"  # generated header files
     DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}"
     COMPONENT ${package_name}-development
 )
 
 # copy target build output artifacts to OS dependent locations
-# (except includes)
+# (except includes, that just sets a compiler flag with the path)
 install(
     TARGETS ${target_name}
     EXPORT ${package_name}-targets
diff --git a/cmake/OptionVariables.cmake b/cmake/OptionVariables.cmake
index 2e95642d1..a7480567a 100644
--- a/cmake/OptionVariables.cmake
+++ b/cmake/OptionVariables.cmake
@@ -1,4 +1,5 @@
-include(GNUInstallDirs)
+# included further down to avoid interfering with our cache variables
+# include(GNUInstallDirs)
 
 
 # ---- Options Summary ----
@@ -93,6 +94,11 @@ if(PROJECT_IS_TOP_LEVEL)
 endif()
 
 
+# do not include earlier or we can't set CMAKE_INSTALL_INCLUDEDIR above
+# required for CMAKE_INSTALL_LIBDIR below
+include(GNUInstallDirs)
+
+
 # ---- Install CMake Directory ----
 
 # This allows package maintainers to freely override the installation path for