Warn on possible mistake in Awk script and Generated C code.

[noyesno]

make test is running clean on FreeBSD now, except for one warning in generated code:

cmp ./backgsub.ok _backgsub && rm -f _backgsub
x.c:269:5: warning: expression result unused [-Wunused-value]
    !strcmp(awka_gets(awka_doln(nf_tight_awk->dval, 0)), "N/A");
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.
cmp ./tweakfld.ok _tweakfld && rm -f _tweakfld

It's due to testcase "tweakfld.awk" has below code. Should consider update the testcase.

The -Wunused-value compiler option is useful. Will consider added as default in awka.

# set TIGHT/LOOSE to N/A if it is not one of the two allowed values
function tight_na() {
    if( $nf_tight !~ /TIGHT|LOOSE/ ) {
        $nf_tight == "N/A"  ;# -- Result NOT Used Here
    }
}

Originally posted by @noyesno in #2 (comment)