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>

configure.ac

@@ -31,7 +31,7 @@ AC_CONFIG_AUX_DIR([config])
 AC_CONFIG_MACRO_DIR([config])
 AC_CANONICAL_SYSTEM
 AM_MAINTAINER_MODE
-AM_SILENT_RULES
+m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])
 AM_INIT_AUTOMAKE([$SPL_META_NAME], [$SPL_META_VERSION])
 AC_CONFIG_HEADERS([spl_config.h], [
 	(mv spl_config.h spl_config.h.tmp &&