In /usr/bin/libpng16-config line 16:
exec_prefix="${prefix}"
^---------^ SC2034: exec_prefix appears unused. Verify use (or export if used externally).


In /usr/bin/libpng16-config line 52:
    exit $1
         ^-- SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
    exit "$1"


In /usr/bin/libpng16-config line 76:
        echo ${ccopts}
             ^-------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
        echo "${ccopts}"


In /usr/bin/libpng16-config line 80:
        echo ${cppflags}
             ^---------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
        echo "${cppflags}"


In /usr/bin/libpng16-config line 84:
        echo ${I_opts} ${cppflags} ${ccopts}
                       ^---------^ SC2086: Double quote to prevent globbing and word splitting.
                                   ^-------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
        echo ${I_opts} "${cppflags}" "${ccopts}"


In /usr/bin/libpng16-config line 88:
        echo >&1 "$(basename $0): --libdir option is disabled in Debian/Ubuntu"
                             ^-- SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
        echo >&1 "$(basename "$0"): --libdir option is disabled in Debian/Ubuntu"


In /usr/bin/libpng16-config line 90:
        echo ${libdir}
             ^-------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
        echo "${libdir}"


In /usr/bin/libpng16-config line 102:
        echo ${L_opts}
             ^-------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
        echo "${L_opts}"


In /usr/bin/libpng16-config line 106:
        echo ${R_opts}
             ^-------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
        echo "${R_opts}"


In /usr/bin/libpng16-config line 110:
        echo ${ldopts}
             ^-------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
        echo "${ldopts}"


In /usr/bin/libpng16-config line 114:
        echo ${ldopts} ${L_opts} ${R_opts} ${libs}
             ^-------^ SC2086: Double quote to prevent globbing and word splitting.
                       ^-------^ SC2086: Double quote to prevent globbing and word splitting.
                                 ^-------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
        echo "${ldopts}" "${L_opts}" "${R_opts}" ${libs}

For more information:
  https://www.shellcheck.net/wiki/SC2034 -- exec_prefix appears unused. Verif...
  https://www.shellcheck.net/wiki/SC2086 -- Double quote to prevent globbing ...