hacking: simplify documentation of bootstrapping process

hacking: simplify documentation of bootstrapping process

* HACKING (Working with git): Do not suggest that, to ensure the
bootstrapping process is performed with the latest autotools, the
developer could explicitly pass $AUTOCONF and $AUTOM4TE in the
environment to the ./bootstrap and ./configure invocations: that
is a little tricky and quite fragile.  Instead, suggest to just
put modern-enough version of the autotools early in PATH.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>