Automake 1.10.1 compat: AM_SILENT_RULES

Automake 1.10.1 compat: AM_SILENT_RULES

Part of the automated testing involves building the source on Debian Lenny
which ships an ancient version of automake (1.10.1).  Historically, this
has caused a non-fatal warning about AM_SILENT_RULES not being defined.
But when the autogen.sh script was updated to use autoreconf the warning
became fatal.

  configure.ac:31: warning: macro `AM_SILENT_RULES' not found in library
  autoreconf: running: /usr/bin/autoconf --force
  configure.ac:34: error: possibly undefined macro: AM_SILENT_RULES
        If this token and others are legitimate, please use m4_pattern_allow.

To resolve this build issue the call to AM_SILENT_RULES has been wrapped
by m4_ifdef().  This prevents the macro from being expanded on platforms
where it's undefined.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>