libpq++ build now checks make rc

I am sorry I did not notice, that libpq++.so does not build correctly on AIX
using gcc,
because it:
1. misses symbols that g++ code needs (libstdc++)
2. the exportfile has demangled C++ symbols (would need nm -C)

So could you please revert that change again, so the build continues.

I cannot fix the problem in time since with all effort I still get:
ld: 0711-345 Use the -bloadmap or -bnoquiet option to obtain more
information.
ld: 0711-317 ERROR: Undefined symbol: __default_alloc_template<false,
0>::_S_end_free
ld: 0711-317 ERROR: Undefined symbol: __default_alloc_template<false,
0>::_S_start_free
ld: 0711-317 ERROR: Undefined symbol: __default_alloc_template<false,
0>::_S_heap_size
ld: 0711-317 ERROR: Undefined symbol: __default_alloc_template<false,
0>::_S_free_list

I guess this problem shows up on all platforms that don't use the compiler
to build libs.
There are some exceptions (irix) in the Makefile, but I think it would need
a more
generic approach.

Andreas