[oe] Why "--disable-perl-regexp" is used for configration in grep

[Pengyu Ma]

Hi Scott and others,

in oe-core grep, perl-regexp is disabled by default:

meta/recipes-extended/grep/grep_2.18.bb:

...

EXTRA_OECONF = "--disable-perl-regexp"

...

Can I know why it is disabled? Any background? I tried to enable it, it 
seems no problem from my side.

Thanks

Pengyu