maint: improve sc_long_lines syntax-check speed

sc_long_lines was the slowest syntax check

before$ time make sc_long_lines
        long_lines
        real    0m2.740s

after $ time make sc_long_lines
        long_lines
        real    0m0.677s

* src/cfg.mk (sc_dd_max_sym_length): s/--max-line-length/-L/
for compat with BSDs.
(sc_long_lines): Prefilter with wc -L to only identify lines
in files that have lines longer than 80 characters.