configure: check for GNU awk, not any awk

Building glibc requires GNU awk, not any other.

Reported-by: Han Sooloo <hansooloo@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

 # Compute Alpha-specific values

 CT_DoArchTupleValues () {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}${CT_ARCH_ALPHA_VARIANT}"

 }