config: hide more config options when a backend

Hide the prompts for some behavioral options, for which the upper-layer build
system is responsible for:
- parallel jobs and maximum load
- use pipes
- use custom shell

 reporter_name="Ioannis E. VENETIS"

 reporter_url="http://sourceware.org/ml/crossgcc/2009-11/msg00109.html"

 reporter_comment=""