tools/automake: do not delete libtool in clean targets

tools/automake: do not delete libtool in clean targets

For the building of some packages, it is helpful
to completely clean the packaged release copy of the source,
however this usually deletes libtool if used,
otherwise causing the need to run the bootstrap script twice.

Not deleting libtool allows compilation to continue directly
after cleaning pre-generated files in the package distribution.

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