Fix non-Python builds

Author: franzpoeschel

CMakeLists.txt

@@ -735,13 +735,17 @@ if(openPMD_USE_INVASIVE_TESTS)
     target_compile_definitions(openPMD PRIVATE openPMD_USE_INVASIVE_TESTS=1)
 endif()
 
-foreach(target openPMD openPMD.py)
+function(set_filesystem_header_for_target target)
     if(openPMD_USE_FILESYSTEM_HEADER)
         target_compile_definitions(${target} PRIVATE openPMD_USE_FILESYSTEM_HEADER=1)
     else()
         target_compile_definitions(${target} PRIVATE openPMD_USE_FILESYSTEM_HEADER=0)
     endif()
-endforeach()
+endfunction()
+set_filesystem_header_for_target(openPMD)
+if(openPMD_HAVE_PYTHON)
+    set_filesystem_header_for_target(openPMD.py)
+endif()
 
 if(openPMD_BUILD_TESTING)
     # compile Catch2 implementation part separately