Support "silent" rules in gnu backend

[vadz]

We should generate the makefile with "silent" rules support, i.e. behaving like Linux kernel or git makefiles do by default and printing just the command and the file it operates on instead of the full command line with all the flags by default -- but enabling the full command display if ran with V=1 command line parameter.

See https://www.gnu.org/software/automake/manual/html_node/Automake-Silent-Rules.html