Merge lp://qastaging/~vanvugt/mir/fix-1609329 into lp://qastaging/mir

Fix a few problems that have prevented cross-compile-chroot.sh from
actually working in a while (LP: #1609329)

  * Fixed error: Some build deps not found (because they are in
    universe rather than main?).
  * Fixed error: Building on yakkety as of recently (today) defaults
    to GCC 6, so the default compiler version when targeting xenial
    or wily now needs to be explicitly set to 5.
  * Fixed error: Precompiled headers from GCC 6 don't work when
    targeting any distro other than yakkety, so don't use them.
  * Fixed error: Some headers from android-headers-23 are missing
    #includes, but apparently only when cross-compiling.
  * Fixed warning: Missing public keys for Ubuntu archive.