autogen.sh: Don't use "egrep -o", use "sed"

autogen.sh: Don't use "egrep -o", use "sed"

"egrep -o" isn't portable and not available on OpenBSD, for example. So
let's use sed instead to get the automake version. The expression used
now is less specific but should work as well ...
(cherry picked from commit 419ff38a07cb0b1637b444c96cd6868a7a9e5524)