configure: also detect plain grep

We need just 'grep', and we need to be able to call it with 'grep -E'.
Check for 'grep', and bailout if egrep != grep -E

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

 #!/bin/sh

 set -e

 printf "Running autoconf...\n"

 autoconf -Wall --force

 printf "Done. You may now run:\n    ./configure\n"