build: detect and create AM_SILENT_RULES macro if needed

Older libs have problem with that:
configure.ac:17: warning: macro `AM_SILENT_RULES'
  not found in library
...
configure.ac:24: error: possibly undefined macro:
   AM_SILENT_RULES
Tested-by: NetDEF CI System <cisystem@netdef.org>
1 file changed