Fix wrong path for slic.cc which has mainpage doc.

config/doxygen/slic_doxygen.cfg.in

@@ -574,7 +574,7 @@ WARN_LOGFILE           =
 # directories like "/usr/src/myproject". Separate the files or directories
 # with spaces.
 
-INPUT                  = ./slic.cc @PROJECT_SOURCE_DIR@/src/ @PROJECT_SOURCE_DIR@/include/
+INPUT                  = @PROJECT_SOURCE_DIR@/slic.cc @PROJECT_SOURCE_DIR@/src/ @PROJECT_SOURCE_DIR@/include/
 
 # This tag can be used to specify the character encoding of the source files
 # that doxygen parses. Internally doxygen uses the UTF-8 encoding, which is