Fix ltdl-libdir test for C++ compilers.

[Ralf Wildenhues]

Pushed as obvious to let 'configure CC=g++' pass the testsuite again.

Cheers,
Ralf

    Fix ltdl-libdir test for C++ compilers.
    
    * tests/ltdl-libdir.at (libdir of installed modules): Declare
    module function as extern "C".

diff --git a/tests/ltdl-libdir.at b/tests/ltdl-libdir.at
index 432d18e..f5dc33c 100644
--- a/tests/ltdl-libdir.at
+++ b/tests/ltdl-libdir.at
@@ -38,7 +38,9 @@ bindir=$prefix/bin
 mkdir $prefix $libdir $bindir
 
 AT_DATA([a.c],
-[[
+[[#ifdef __cplusplus
+extern "C"
+#endif
 int f (void)
 {
   return 42;