[maint-guide] make -j1

[Osamu Aoki <osamu@debian.org>]

Hi, Raphael, I appreciate your efforts.

I think this is better discussed publicly for the record for
maint-guide.

I understand your feeling that -j1 option is spurious to "make".

But these are there in real life if you run current debhelper dh_auto_*.
I tested these with test packages Until then, I did not have these
here).  These values can be changed by DEB_BUILD_OPTIONS when --parallel
is used.

   --parallel
       Enable parallel builds if underlying build system supports them.
       The number of parallel jobs is controlled by the DEB_BUILD_OPTIONS
       environment variable ("Debian Policy, section 4.9.1") at build
       time. It might also be subject to a build system specific limit.

I am leaning to ward reverting these changes while adding small footnote to
lead people to this fact.  These seems trivial point but it is important
when people start debugging for parallel build issues.  

Osamu