configure.ac: Fix stupid autotools errors

Apparently, recent versions of autotools will issue an error if an empty
description is supplied to AC_DEFINE(). Avoid these errors by just
adding a space in the square brackets.

Partially fixes Bugzilla #4908.

--HG--
extra : rebase_source : c2f52ec795e159d18a0082df012ff30d6bb2e3e0
1 file changed