Silence Automake to reduce size of build logs

In my test, this reduced log size from 4.2MB to 3.2MB.

Example of the new output (taken from Gem compilation):

  CXX      libplugins_la-record.lo
record.cpp:58:3: warning: #warning better handling of duplicate codecs [-Wcpp]
 # warning better handling of duplicate codecs
   ^~~~~~~
record.cpp:87:3: warning: #warning isThreadable [-Wcpp]
 # warning isThreadable
   ^~~~~~~
record.cpp:257:3: warning: #warning enumerateProperties [-Wcpp]
 # warning enumerateProperties
   ^~~~~~~
  CXX      libplugins_la-video.lo
  CXX      libplugins_la-videoBase.lo
  CXXLD    libplugins.la

Not all components honour the V=0 flag -- perhaps they don't use Automake, or use a very old version. However, many do.

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

See also: #666