+ # Don't explore the same directory multiple times. This is here not
+ # only for speedup purposes. We need this when the user has e.g.
+ # specified 'ACLOCAL_AMFLAGS = -I m4' and has also set
+ # AC_CONFIG_MACRO_DIR[S]([m4]) in configure.ac. This makes the 'm4'
+ # directory to occur twice here and fail on the second call to
+ # scan_m4_dirs([m4]) when the 'm4' directory doesn't exist.
+ # TODO: Shouldn't there be rather a check in scan_m4_dirs for
+ # @user_includes[0]?
+ @user_includes = uniq @user_includes;
+