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

[Akim Demaille]

 commit 0e524d979be008dc083884fb284d8de5b2355507
 Author: Akim Demaille <address@hidden>
 Date:   Fri Sep 7 13:59:47 2018 +0200

    configure: reveal the name of the Valgrind suppression file we use
    
    * configure.ac: here.
    * build-aux/Linux.valgrind (libstdcxx_init): New.

diff --git a/build-aux/Linux.valgrind b/build-aux/Linux.valgrind
index a40719fb..e9cb906c 100644
--- a/build-aux/Linux.valgrind
+++ b/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
 {
diff --git a/configure.ac b/configure.ac
index 94fab063..2ed6f0b1 100644
--- a/configure.ac
+++ b/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])