autogen.sh: remove workaround for automake <1.14, needed for EL <=7

Ultimately this is a revert of 779ac93, which according to
@nabijaczleweli is to paper over automake <1.14's lack of
%reldir% support.

As I understand it, EL8 is the lowest current build target.

Reviewed-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Joseph Holsten <joseph@josephholsten.com>
Closes #17878