Split the process to fetch GNU tools (until now embedded
within tools/Makefile.gnuhost) into a new Makefile.fetchgnu,
MINIX-specific hence relocated, which is to be also used
to fetch sources even when not building the tools.
Use it for binutils too.
Improve documentation.
Also do not run configure on each run when MKUPDATE=yes
The .WAIT serialization instruction between fetching and other
configure sources was raising a new run of configure at each
compilation. Avoid it by using two rules.