build: Terminate lists in variables with «# EOL»

This means we can add a trailing «\» to every element, so that they
can be removed without requiring modification of other lines, and can
be easily sorted.

Replace the old usage of $(nil) which could possibly end up with junk
added if such variable is ever defined, in the environment.

include/Makefile.am

@@ -29,4 +29,4 @@ nobase_include_HEADERS = \
 	bsd/unistd.h \
 	bsd/vis.h \
 	bsd/wchar.h \
-	$(nil)
+	# EOL