don't look for a file called 'concepts' as input. It doesn't exist

[[Imported from SVN: r6404]]

doc/doxygen/Doxylocal

 # Where to search and which files to use
 INPUT                 += @srcdir@/mainpage \
                          @srcdir@/modules \
-                         @srcdir@/concepts \
                          @top_srcdir@/dune/common/modules \
                          @top_srcdir@/dune/common
 EXCLUDE               += @top_srcdir@/dune/common/test