avoid egrep obsolecent warning by switching to grep -E.

[ratmice]

At some point GNU grep decided to deprecate the egrep utility in favor of grep -E.

# Full path to egrep given to avoid shell aliases.
$ /usr/bin/egrep 'whatever' /dev/null
egrep: warning: egrep is obsolescent; using grep -E

I haven't tested building on other platforms/operating systems, but given that posix grep does define
the -E argument to grep, but does not appear to require an egrep command it seems like
one should prefer it.

https://pubs.opengroup.org/onlinepubs/9799919799/utilities/grep.html#top

[ppannuto]

I just checked, and this does work out of the box on a current OS X install.