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 <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/16522
Signed-off-by: Robert Marko <[email protected]>

tools/automake/patches/999-clean-am.patch

@@ -18,3 +18,11 @@
  
  .PHONY: clean mostlyclean distclean maintainer-clean \
  clean-generic mostlyclean-generic distclean-generic maintainer-clean-generic
+--- a/lib/am/libtool.am
++++ b/lib/am/libtool.am
+@@ -25,4 +25,4 @@ clean-libtool:
+ 
+ ?TOPDIR_P?distclean-am: distclean-libtool
+ ?TOPDIR_P?distclean-libtool:
+-?TOPDIR_P?	-rm -f libtool config.lt
++?TOPDIR_P?	-touch libtool config.lt