cmake: fix parallel build of doc

We found a race condition during parallel build (-j48) on Fedora for the docs.

docs/CMakeLists.txt

@@ -37,8 +37,10 @@ set(DOXYGEN_EXAMPLE_PATH ${PROJECT_SOURCE_DIR}/example)
 # configure doxygen config file
 configure_file(${DOXYFILE_IN} ${DOXYFILE_OUT} @ONLY)
 
-# create doxygen output directory
+# create doxygen output directories - to avoid parallel build issue
 file(MAKE_DIRECTORY ${DOXYGEN_OUTPUT_DIR})
+file(MAKE_DIRECTORY ${DOXYGEN_OUTPUT_DIR}/html)
+file(MAKE_DIRECTORY ${DOXYGEN_OUTPUT_DIR}/xml)
 
 # todo(s.schulz): Add dependency on other files (Fortran module etc.)
 add_custom_command(