libgnutlsxx link problems

[Boyan Kasarov]

Hello,
I am sending two patches to add symbol visibility support.

[PATCH 1/2] Import lib-symbol-visibility from gnulib
[PATCH 2/2] Use lib-symbol-visibility for the C++ library

The first patch is the result of running:
# gnulib-tool --import lib-symbol-visibility
from latest gnulib from git, but I think it also updated some other files, if you have your ways to invoke it, please feel free to do it.

The second patch adds the necessary changes as explained in lib-symbol-visibility manual to add support for libgnutlsxx. I also removed the "local: *;" directive from libgnutlsxx.map, because it is no longer needed to hide symbols using it.

I have tested on freebsd, linux and opensolaris with gcc 4.2, 4.3 and 3.4.

Boyan