use 'or' instead of '||' as operator

this fixes a stupid FTBFS with mawk which thinks our file contains regex:

    NP_VERSION=1.5.110.g2d494 mawk -f ./subst check_mailq.pl > check_mailq
    mawk: run time error: regular expression compile failed (missing operand)
    /var/lib/postfix' || -d '/var/local/lib/postfix
    	FILENAME="check_mailq.pl" FNR=614 NR=614
    make[2]: *** [check_mailq] Error 2