[autotools,bugfix] Adds share/dune/aclocal to the include path of autogen.

Previously aclocal did not find the DUNE macros when they were installed to
the new location. This patch fixes this by adding the path to this list of
include paths if it is available.

bin/dune-autogen

@@ -68,6 +68,9 @@ for OPT in "$@"; do
               echo "Found am directory $OPT/am"
               am_dir="$OPT/am"
             fi
+            if test -d "$OPT/share/dune/aclocal"; then
+              ACLOCAL_FLAGS="$ACLOCAL_FLAGS -I $OPT/share/dune/aclocal"
+            fi
             if test -d "$OPT/share/aclocal"; then
               ACLOCAL_FLAGS="$ACLOCAL_FLAGS -I $OPT/share/aclocal"
             fi