Makefile読めばわかるだろう...

# A normal qmail installation puts everything into /var/qmail/.
# If you want to install to /usr/local/, then "/usr/local/qmail" is
# suggested instead of "/usr/local", but both will work.
PREFIX?= /var/qmail
QMAIL_VERSION?= 1.03

本当の正解はqmailを使わない