Move the extra configure flags for cross building out from the build side code and instead
prepend them to the configure-flags.
Use new procedure cmake-system-name-for-target to add support for hurd and
bare-metal targets.
* guix/build/cmake-build-system.scm (configure): Move cross build flags from
here ...
* guix/build-system/cmake.scm (cmake-cross-build): ... to here.
(cmake-system-name-for-target): New procedure.
Change-Id: Ic68acc246e543491ed147e53d47cec5de46b82cb
Signed-off-by: Ludovic Courtès <ludo@gnu.org>