Update mesa to 11.0.8.
- Depend on libelf on DragonFly
- Rework the vdpau support code and enable vdpau for DragonFly, FreeBSD doesn't support it just yet.
- Revert commit that removed support for non-render nodes. Neither FreeBSD nor DragonFly supports render nodes at this time.
- Fix LDFLAGS to work with GCC5, which DragonFly uses by default.
- Update Makefile.targets: ffast-math isn't used since 8.0.3, don't replace x86_64 with amd64 on Dragonfly. Drop obsolete python disable replace.