Merge remote-tracking branch 'remotes/bonzini/configure' into staging

Merge remote-tracking branch 'remotes/bonzini/configure' into staging

* remotes/bonzini/configure:
  rules.mak: Rewrite unnest-vars
  configure: unset interfering variables
  configure: duplicate/incorrect order of -lrt
  libcacard: improve documentation
  libcacard: actually use symbols file
  libcacard: replace qemu thread primitives with glib ones
  vscclient: use glib thread primitives not qemu
  glib-compat.h: add new thread API emulation on top of pre-2.31 API

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>