Mark system (external) header files correctly

Use -isystem to suppress warnings from headers

Framework/Geometry/CMakeLists.txt

@@ -44,8 +44,14 @@ target_link_libraries (
 
 target_include_directories (
   CORSIKAgeometry
+  SYSTEM
   PUBLIC    ${EIGEN3_INCLUDE_DIR}
   INTERFACE ${EIGEN3_INCLUDE_DIR}
+  )
+
+target_include_directories (
+  CORSIKAgeometry
+  INTERFACE
   $<BUILD_INTERFACE:${PROJECT_BINARY_DIR}/include>
   $<INSTALL_INTERFACE:include/include>
   )

Processes/Pythia/CMakeLists.txt

@@ -48,17 +48,12 @@ target_include_directories (
   $<BUILD_INTERFACE:${PROJECT_BINARY_DIR}/include>
   $<INSTALL_INTERFACE:include/include>
   )
-# do it again, since we want -isystem for pythia exteranls
-target_include_directories (
-  ProcessPythia
-  SYSTEM PUBLIC
-  ${PYTHIA8_INCLUDE_DIR}
-  )
 
 target_include_directories (
   ProcessPythia
-  PRIVATE
-  ${PYTHIA8_INCLUDE_DIR}
+  SYSTEM
+  PUBLIC ${PYTHIA8_INCLUDE_DIR}
+  INTERFACE ${PYTHIA8_INCLUDE_DIR}
   )
 
 install (