.gitignore: do not ignore custom files in m4

Ignoring the subtree m4/ cannot be overridden by subsequent rules.
To include the ax_*.m4 macro sets, the auto-generated files to be
ignored in the m4 directory must be listed explicitly.

This change addresses bug #58.

.gitignore

@@ -17,9 +17,11 @@ install-sh
 libcoap-*.tar.bz2
 libtool
 ltmain.sh
-m4/
-# warn about specific macros in the m4 directory if they missing
-!m4/ax_*
+m4/libtool.m4
+m4/ltoptions.m4
+m4/ltsugar.m4
+m4/ltversion.m4
+m4/lt~obsolete.m4
 missing
 stamp-h1