Avoid warnings from recent versions of doxygen

doc/doxygen/doxygen.cfg

@@ -365,7 +365,7 @@ TYPEDEF_HIDES_STRUCT   = NO
 # 2^(16+SYMBOL_CACHE_SIZE). The valid range is 0..9, the default is 0,
 # corresponding to a cache size of 2^16 = 65536 symbols.
 
-SYMBOL_CACHE_SIZE      = 0
+#SYMBOL_CACHE_SIZE      = 0
 
 # Similar to the SYMBOL_CACHE_SIZE the size of the symbol lookup cache can be
 # set using LOOKUP_CACHE_SIZE. This cache is used to resolve symbols given
@@ -1502,13 +1502,13 @@ XML_OUTPUT             = xml
 # which can be used by a validating XML parser to check the
 # syntax of the XML files.
 
-XML_SCHEMA             =
+#XML_SCHEMA             =
 
 # The XML_DTD tag can be used to specify an XML DTD,
 # which can be used by a validating XML parser to check the
 # syntax of the XML files.
 
-XML_DTD                =
+#XML_DTD                =
 
 # If the XML_PROGRAMLISTING tag is set to YES Doxygen will
 # dump the program listings (including syntax highlighting

doc/doxygen_dev/doxygen.cfg

@@ -365,7 +365,7 @@ TYPEDEF_HIDES_STRUCT   = NO
 # 2^(16+SYMBOL_CACHE_SIZE). The valid range is 0..9, the default is 0,
 # corresponding to a cache size of 2^16 = 65536 symbols.
 
-SYMBOL_CACHE_SIZE      = 0
+#SYMBOL_CACHE_SIZE      = 0
 
 # Similar to the SYMBOL_CACHE_SIZE the size of the symbol lookup cache can be
 # set using LOOKUP_CACHE_SIZE. This cache is used to resolve symbols given
@@ -1502,13 +1502,13 @@ XML_OUTPUT             = xml
 # which can be used by a validating XML parser to check the
 # syntax of the XML files.
 
-XML_SCHEMA             =
+#XML_SCHEMA             =
 
 # The XML_DTD tag can be used to specify an XML DTD,
 # which can be used by a validating XML parser to check the
 # syntax of the XML files.
 
-XML_DTD                =
+#XML_DTD                =
 
 # If the XML_PROGRAMLISTING tag is set to YES Doxygen will
 # dump the program listings (including syntax highlighting