[Bug #19778] Add `-I` options for opt-dir to `$INCFLAGS`

These options have been separated from `$CFLAGS` already in the
other places.

configure.ac

@@ -967,7 +967,7 @@ AS_IF([test "x$OPT_DIR" != x], [
     LDFLAGS="${LDFLAGS:+$LDFLAGS }$val"
     DLDFLAGS="${DLDFLAGS:+$DLDFLAGS }$val"
     LDFLAGS_OPTDIR="$val"
-    CPPFLAGS="${CPPFLAGS:+$CPPFLAGS }"`echo "$OPT_DIR" | tr "${PATH_SEPARATOR}" '\012' |
+    INCFLAGS="${INCFLAGS:+$INCFLAGS }"`echo "$OPT_DIR" | tr "${PATH_SEPARATOR}" '\012' |
         sed '/^$/d;s|^|-I|;s|$|/include|' | tr '\012' ' ' | sed 's/ *$//'`
 ])