Child processes should be session leaders.

If not, when the session of mu is killed, these child processes are also
killed. This scenario shows up when using mu4e: a PDF attachment, for
example, is opened by Evince, but as soon as Emacs exits, Evince is also
killed.

configure.ac

@@ -122,7 +122,7 @@ AS_IF([test "x$ac_cv_member_struct_dirent_d_ino" != "xyes"],
 
 
 # we need these
-AC_CHECK_FUNCS([memset memcpy realpath setlocale strerror getpass])
+AC_CHECK_FUNCS([memset memcpy realpath setlocale strerror getpass setsid])
 AC_CHECK_FUNC(timegm,[],[AC_MSG_ERROR([missing required function timegm])])
 
 # require pkg-config