autoconf: Mangle to use /usr/bin/env perl

autoconf: Mangle to use /usr/bin/env perl

On the host side we can have the full path exceed the limit for the #!
method.  On the target side, we already had to play a game to ensure
the path would be valid.  So we change to using /usr/bin/env there
as well, which is safe.

Signed-off-by: Tom Rini <tom_rini@mentor.com>