makefiles: stop failing when config.mk does not exist

This allows running `make clean` and `make distclean` (and possibly
others) without having to run ./configure beforehand.

Note that some packaging-related targets still depend on the existence
of generated files.  For example:

* dist: config.mk
* deb: config.sh

Commands used to search and replace:

    $ git grep -Elz 'include *([^ ]*/)?config.mk' | xargs -0 -I '{}' \
      sh -c "printf '%s\n' \
      \"\$(sed -E 's|^include *(([^ ]*/)?config.mk)|-include \1|' '{}')\" >'{}'"

Relates to #5140.

1 files changed, 1 insertions, 1 deletions

diff --git a/Makefile b/Makefile
index c235aff7c..5d14ce467 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,4 @@
-include config.mk
+-include config.mk
 
 ifneq ($(HAVE_MAN),no)
 MAN_TARGET = man