work around Doxygen bug triggered by r814874, which caused many classes to disappear

from the docs.

doc/Doxyfile.in

@@ -1174,7 +1174,8 @@ INCLUDE_FILE_PATTERNS  =
 # undefined via #undef or recursively expanded use the := operator 
 # instead of the = operator.
 
-PREDEFINED             = EIGEN_EMPTY_STRUCT
+PREDEFINED             = EIGEN_EMPTY_STRUCT \
+                         EIGEN_PARSED_BY_DOXYGEN
 
 # If the MACRO_EXPANSION and EXPAND_ONLY_PREDEF tags are set to YES then 
 # this tag can be used to specify a list of macro names that should be expanded. 

doc/Mainpage.dox

@@ -12,4 +12,4 @@ The many other classes are typically return types for MatrixBase methods.
 
 */
 
-}
+}