Introducing support for POSIX-nm, required for AIX.

Hi!

On AIX, when _GLOBAL__* symbols are exported from shared libraries,
and the final executable is linked with runtime linking enabled,
GNU C++ exception handling may break.

Unfortunately, the command to create the list of exported symbols
for AIX is quite complicated at the moment, not only because it
relies on the AIX-nm to be run in POSIX mode.

So the first patch does add generic support for POSIX-nm to libtool,
and simplifies the export_symbols_cmds for AIX a lot.

Finally, the second patch does actually stop exporting above symbols.

Although this topic is independent of the patch series titled
"AIX: Do not rely on OBJECT_MODE env var being set." [1]
(no response yet;), this patch series is based on that one, as both
deal with NM flags.

[1] http://thread.gmane.org/gmane.comp.gnu.libtool.patches/11819

Thanks!
/haubi/