libc/glibc: run ./configure in CONFIG_SHELL

Tremendously helps when running on at least Ubuntu, with dash as
the system shell (ie. /bin/sh points to dash).

Reported by a few people, of which:
leming, ccct and ccole on IRC

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 reporter_name="Kevin Cernekee"

 reporter_url="http://sourceware.org/ml/crossgcc/2011-09/msg00037.html"

 reporter_comment="MIPS SDE-compatible toolchain"