Adds initial intermediate qt6 porting state

qt6-after/qt6-migrated-legatus/cmake/DoxygenMacros.cmake

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+FIND_PROGRAM(DOXYGEN_EXECUTABLE doxygen)
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+MACRO(ADD_DOXYGEN_TARGET target input output infile outdir)
+  IF(DOXYGEN_EXECUTABLE-NOTFOUND)
+    MESSAGE(STATUS "Doxygen executable not found - can't create doxygen documentation")
+  ELSE(DOXYGEN_EXECUTABLE-NOTFOUND)
+    CONFIGURE_FILE(${input} ${output})
+    ADD_CUSTOM_TARGET(doxygen_${target} doxygen ${output}
+      COMMAND ${CMAKE_COMMAND} -E copy_if_different ${infile} ${outdir}
+    )
+  ENDIF(DOXYGEN_EXECUTABLE-NOTFOUND)
+ENDMACRO(ADD_DOXYGEN_TARGET target)