Use the proper C++ compiler to run C++ tests

Running the C compiler with -shared-libgcc -lstdc++ does not work on
non-GCC compilers.

configure.ac

@@ -29,6 +29,7 @@ m4_rename([_AC_ARG_VAR_PRECIOUS],[real_PRECIOUS])
 m4_define([_AC_ARG_VAR_PRECIOUS],[])
 save_CFLAGS=$CFLAGS
 AC_PROG_CC
+AC_PROG_CXX
 CFLAGS=$save_CFLAGS
 m4_undefine([_AC_ARG_VAR_PRECIOUS])
 m4_rename_force([real_PRECIOUS],[_AC_ARG_VAR_PRECIOUS])
@@ -57,6 +58,7 @@ fi
 
 cat > local.exp <<EOF
 set CC_FOR_TARGET "$CC"
+set CXX_FOR_TARGET "$CXX"
 EOF
 
 AM_MAINTAINER_MODE