Fix bashism in configure test

Fix the use of "==" operator in configure script that is bashism. This operator is incorrect in non-bash shells (such as dash), and causes the subsequent check never to be executed, effectively trying to build xfburn without gst-pbutils.

Originally reported as https://bugs.gentoo.org/917576

requested review from @huntertur

approved this merge request

merged

Thank you @mgorny; good catch by you and the Gentoo folks!

Thanks!