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>

 reporter_name="dsreed on freenode/#crosstool-ng"

 reporter_url=""

 reporter_comment="Enables experimental support for VFP/NEON hardware floating point."