configure: do not fork off grep subprocess in probe_cc

grep is not required for the functionality in this instance.
This avoids an unnecessary fork, and also avoids a duplicated dumpversion call.
Furthermore, it also corrects behavior when no minor version number is present, see e.g
https://github.com/joyent/node/pull/25671

.

Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>