This is considered to be more robust.
Two instances remain:
scripts/build/chroot_archives, line 257:
if [ "${LB_APT}" = "aptitude" ] && [ ! $(Chroot chroot "which aptitude") ]
The command is run inside a chroot where the environment might be special,
and would need further testing.
manpages/Makefile, line 42:
@if [ ! -x "$$(which po4a 2>/dev/null)" ]; \
I am insufficiently familiar with makefile syntax to edit this.