Improved ignore for 'm4' macro directory

The previous rule was to ignore the whole directory, while still having
some files tracked inside.

Now we only explicitly ignore the files that have been placed there by the
autotools, so we can have a better view of it in Git when adding stuff.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>

.gitignore

@@ -22,7 +22,12 @@ ltmain.sh
 missing
 mkinstalldirs
 stamp-h1
-m4/
+m4/ld-version-script.m4
+m4/libtool.m4
+m4/ltoptions.m4
+m4/ltsugar.m4
+m4/ltversion.m4
+m4/lt~obsolete.m4
 
 src/wconfig.h