shellcheck: scan-build.sh issues

Shellcheck (​https://github.com/koalaman/shellcheck) finds the following issues:

In scan-build.sh line 37:
EXTRA_CHECKERS="\
^-- SC2034: EXTRA_CHECKERS appears unused. Verify use (or export if used externally).


In scan-build.sh line 44:
NOISY_CHECKERS="\
^-- SC2034: NOISY_CHECKERS appears unused. Verify use (or export if used externally).


In scan-build.sh line 56:
    $CHECKERS \
    ^-- SC2086: Double quote to prevent globbing and word splitting.


In scan-build.sh line 65:
    $CHECKERS \
    ^-- SC2086: Double quote to prevent globbing and word splitting.


In scan-build.sh line 70:
    $CHECKERS $OUTPUTARG \
    ^-- SC2086: Double quote to prevent globbing and word splitting.