ignore m4/ directory and (re)create it in autogen.sh

m4/ is used by auto* tools to store (mostly libtool)
generated files.  Without it being present, autoreconf&Co
does not work, and with it present, git mentions untracked
files.  Fix both.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

.gitignore

@@ -2,6 +2,7 @@
 /depcomp
 /aclocal.m4
 /missing
+/m4/
 /config.guess
 /config.sub
 /install-sh