Search more library dirs with --enable-shared

[Alexandre Oliva]

This patch arranges for the top-level Makefile to set LD_LIBRARY_PATH
or equivalent such that the directory in which a shared C++ library
would live.  This is important to get libgcj's configure tests (and
any other C++ library's, for that matter) to succeed, when shared
libraries are enabled.

I'm checking this in.  Tested on i686-pc-linux-gnu.