Replace m4/.keep file with .gitignore one.

* m4/.keep: Rename to m4/.gitignore; update the content.
* .gitignore: Replace multiple entries in m4/ with a wildcard.

.gitignore

@@ -50,11 +50,7 @@ Makefile.in
 # Generated by libtoolize:
 /libtool
 /ltmain.sh
-/m4/libtool.m4
-/m4/ltoptions.m4
-/m4/ltsugar.m4
-/m4/ltversion.m4
-/m4/lt~obsolete.m4
+/m4/*.m4
 
 # These files are generated by make check:
 /tests/list_atomic.c

m4/.gitignore

@@ -0,0 +1,3 @@
+# Place holder to keep this directory in the Git repository.
+*
+!.gitignore