Debian uses dash by default, which doesn't do '+='.  Fixes servo's
osmesa-based headless testing system, which was looking for libOSMesa in
the lib/ directory.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Cc: mesa-stable@lists.freedesktop.org