I have previously installed gfortran using my package manager (apt-get
on Ubuntu 10.02 [Lucid]).  Today I tried to build a fresh copy of
gcc-4.6.3 by downloading the tarball, unzipping it, and running
configure in a separate directory.

e.g.

This all works fine (I get ~/bin/gfortran-4.6.3) which seems to
*work*.  (It produces an executable on a very simple test program).
The problem is when I try to run the test program.

First, the runtime library complains that it can't find libquadmath.so.
0 ...

Naively, I soft linked the one that I found in $(build_dir)/x86_64-
unknown-linux-gnu/libquadmath/.libs/libquadmath.so to /lib/.

Now I have the error:

./junk.exe: /usr/lib/libgfortran.so.3: version `GFORTRAN_1.4' not
found (required by ./junk.exe)

I could again symlink the library from the build-tree (and so on), but
I'm cautious to do that since I don't want to replace any of the
runtime libraries my old version of gfortran relies on.

Is there a better way to do this?  What am I missing here?
(Of course, I could just set LD_LIBRARY_PATH -- but the install
doesn't seem to have installed all of the libraries anywhere ... The
all just live in /path/to/.libs/lib*.so* ...)

Any help would be greatly appreciated.