tools/automake: do not delete Makefile in clean targets

For the building of some packages, it is sometimes helpful
to completely clean the packaged release copy of the source,
however this usually both requires and deletes the Makefile,
causing the need to run the configure stage twice.

Not deleting the Makefile 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>

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

@@ -0,0 +1,20 @@
+--- a/lib/am/clean.am
++++ b/lib/am/clean.am
+@@ -39,7 +39,7 @@ distclean-generic:
+ ## If you change distclean here, you probably also want to change
+ ## maintainer-clean below.
+ distclean:
+-	-rm -f %MAKEFILE%
++	-touch %MAKEFILE%
+ 
+ maintainer-clean-am: maintainer-clean-generic distclean-am
+ maintainer-clean-generic:
+@@ -51,7 +51,7 @@ maintainer-clean-generic:
+ 
+ ## See comment for distclean.
+ maintainer-clean:
+-	-rm -f %MAKEFILE%
++	-touch %MAKEFILE%
+ 
+ .PHONY: clean mostlyclean distclean maintainer-clean \
+ clean-generic mostlyclean-generic distclean-generic maintainer-clean-generic