audio/baresip: Honor GL_DEFAULT, use opt_USE

Replace "opt_VARS= USE_xx+=yy" with "opt_USE= XX=yy".

PR: 293131
Approved by: Marek Zarychta <zarychtam@plan-b.pwste.edu.pl> (maintainer)
Co-authored-by: Eric Camachat <eric@camachat.org>