Add experimental -rpath to the LDFLAGS
Split the dual-mode PG::Connection#exec into #exec and #exec_params.

PG::Connection#exec should be backward compatible with old code: it just passes
control to #exec_params directly instead of calling PQexecParams() itself.

Thanks to Aaron Patterson for the idea.
Avoid warning about DWORD to int mapping in rb_raise() format string.
Merge with rpath_ldflag