configure: reveal the name of the Valgrind suppression file we use

* configure.ac: here.
* build-aux/Linux.valgrind (libstdcxx_init): New.

build-aux/Linux.valgrind

@@ -15,6 +15,19 @@
 
 # Valgrind suppression file for Bison.
 
+# Travis Trusty, 2018-09-07.
+{
+   libstdcxx_init
+   Memcheck:Leak
+   match-leak-kinds: reachable
+   fun:malloc
+   obj:/usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.25
+   fun:call_init.part.0
+   fun:call_init
+   fun:_dl_init
+   obj:/lib/x86_64-linux-gnu/ld-2.19.so
+}
+
 # Linux prague 4.1.2-2-ARCH #1 SMP PREEMPT Wed Jul 15 08:30:32 UTC 2015
 # x86_64 GNU/Linux
 {

configure.ac

@@ -245,6 +245,8 @@ case $VALGRIND:$uname in
     fi
     ;;
 esac
+AC_MSG_CHECKING([Valgrind suppression file])
+AC_MSG_RESULT([$suppfile])
 
 # Whether we cannot run the compiled bison.
 AM_CONDITIONAL([CROSS_COMPILING], [test "$cross_compiling" = yes])