Before the code would fail silently for zero values and with some output for
empties. We now currently handle both via defaulting value to zero and making
`let` return success error code when there's no syntax error.
Now is an opportune time to do this, as the infixSalt conversion in
`add-flags.sh` ensures that all the relevant `NIX_*` vars will be
defined even if empty.
This makes the response file handling more consistent with GCC.
For example, a reponse file may contain:
"-Wl,$ORIGIN"
GCC will treat this as a double quoted string and not expand the
variable reference. Previously, cc-wrapper would expand the variable
in the same was as if the string was provided on the command line.
