The interaction of environment variables and make variables is fun.

For some reason, the
workdir/.../UnpackedTarball/nss/mozilla/nsprpub/configure script is
run twice: Once directly from nss/ExternalProject_nss.mk, once from
the sub-make run from nss/ExternalProject_nss.mk. In the first case,
the AR and RANLIB exported by the gbuild make process propagate just
fine to the configure script. In the latter case, not. So add AR and
RANLIB assignments on the sub-make command line (to override values
set in some of the nss makefiles), *and* make sure the sub-make
exports AR and RANLIB.

Change-Id: Ibd55bc8a7e001106e12b2207500e74c7bd01c73a