Add doxygen template to CMake docs target.

- Adds the template file to the target enabling easy
  edit via IDE editor.
- Drop a generated file comment at the beginning of
  the generated doxyfile in hopes of informing users it
  should not be modified.

BUG=aomedia:76,aomedia:559

Change-Id: Id4e2762a5dae145927e7b7f1f6689744150ba090

docs.cmake

@@ -202,10 +202,9 @@ reference. The following utilities are included:
   set(AOM_DOXYGEN_SOURCES ${AOM_DOXYGEN_SOURCES} ${samples_dox})
 
   # Generate libaom's doxyfile.
-  file(COPY "${AOM_ROOT}/${AOM_DOXYGEN_CONFIG_TEMPLATE}"
-       DESTINATION "${AOM_CONFIG_DIR}")
-  file(RENAME
-       "${AOM_CONFIG_DIR}/${AOM_DOXYGEN_CONFIG_TEMPLATE}" "${AOM_DOXYFILE}")
+  file(WRITE "${AOM_DOXYFILE}" "##\n## GENERATED FILE. DO NOT EDIT\n##\n")
+  file(READ "${AOM_ROOT}/${AOM_DOXYGEN_CONFIG_TEMPLATE}" doxygen_template_data)
+  file(APPEND "${AOM_DOXYFILE}" ${doxygen_template_data})
   file(APPEND "${AOM_DOXYFILE}"
        "EXAMPLE_PATH += ${AOM_ROOT} ${AOM_ROOT}/examples\n")
   file(APPEND
@@ -221,8 +220,10 @@ reference. The following utilities are included:
                     COMMAND "${DOXYGEN_EXECUTABLE}" "${AOM_DOXYFILE}"
                     DEPENDS "${AOM_DOXYFILE}" ${AOM_DOXYGEN_SOURCES}
                              ${AOM_DOXYGEN_EXAMPLE_SOURCES}
+                             "${AOM_DOXYGEN_CONFIG_TEMPLATE}"
                     SOURCES "${AOM_DOXYFILE}" ${AOM_DOXYGEN_SOURCES}
-                             ${AOM_DOXYGEN_EXAMPLE_SOURCES})
+                             ${AOM_DOXYGEN_EXAMPLE_SOURCES}
+                             "${AOM_DOXYGEN_CONFIG_TEMPLATE}")
 endfunction ()
 
 endif ()  # AOM_DOCS_CMAKE_