tools/libtool: add maintainer-clean to clean target

tools/libtool: add maintainer-clean to clean target

Use the clean recipe during build, so that files already
generated by the bootstrapping within the release are removed
before the bootstrap script is run again.

Override an automake variable "am__CONFIG_DISTCLEAN_FILES"
and related variables for their dependencies
in order to prevent removing config.status and other configure files
so that configure stage does not need to be ran twice.

Automake with modified rules must be ran
in order to generate a Makefile that does not delete itself.

Signed-off-by: Michael Pratt <mcpratt@pm.me>
Link: https://github.com/openwrt/openwrt/pull/16522
Signed-off-by: Robert Marko <robimarko@gmail.com>