Prevent propagation of user flags to bundled autotools projects

autotools allows to set site-wide flags to configure by reading from the
file pointed to by the environment variable CONFIG_SITE, see e.g.

  https://www.gnu.org/software/automake/manual/html_node/config_002esite.html

Since we try to set consistent compiler and linker flags in bundled
autotool projects ourself, letting flags from CONFIG_SITE leak can lead
to an inconsistent build (e.g. `./configure` on a 64-bit platform and
CONFIG_SITE with CXXFLAGS=-m32).

Since the configure of bundled projects is run in the build and not the
configure phase unset CONFIG_SITE prevents propagation of any user
flags.

Signed-off-by: Fons Rademakers <Fons.Rademakers@cern.ch>
update for iOS 7 support.