configure: allow environments variable to override internals

configure checks for grep, egrep, sed and ar and set the variables GREP,
EGREP, SED and AR accordingly. We now let already set variables override
the internal choices to let users make decisions when they know the
right choice already. This is a regression as our configure script used
to allow this back before commit 0b57c475 (up to 7.18.2).

Reported by: "kdekker"
Bug: http://curl.haxx.se/bug/view.cgi?id=3028318