If the compiler dumps core, bash will print a useless error
message with the full path of the errchk script. Catch that
by wrapping the if statement which invokes the compiler in a
subshell. Use the $TMPOUT file as a flag to let the main
shell know whether the subshell ran. Since the compiler
stdout and stderr are redirected, if the if statement produces
any output, then the compiler crashed, and we report that.