include headers twice to check for header guards

cmake/TestHeaders.cmake

@@ -115,6 +115,7 @@ function(add_header_test target)
         set(source "${CMAKE_CURRENT_BINARY_DIR}/headers/${directory}/${filename}.cpp")
         if(NOT EXISTS "${source}")
             file(WRITE "${source}" "#include <${header}>")
+            file(WRITE "${source}" "#include <${header}>") # do it twice to ensure that header guards are provided
         endif()
         list(APPEND sources "${source}")
     endforeach()