[PATCH 0/3, updated] AIX-driven support for POSIX nm

Hi,

've reworked the AIX driven patches based on the discussion around
patch "[3/4] Use POSIX nm to simplify AIX export_symbols_cmds."
http://lists.gnu.org/archive/html/libtool-patches/2016-03/msg00016.html

Previous patch "[1/4] Fix func_echo_all inside configure." is not
required any more, although it still makes sense to be committed.

Previous patch "[2/4] AIX: Do not rely on OBJECT_MODE env var being set."
now is "[1/3] AIX: use proper ABI flags in search for toolchain" and
requires the gnulib patch "early check for PowerPC AIX object mode",
http://lists.gnu.org/archive/html/bug-gnulib/2016-03/msg00046.html
although that one still is waiting for attention by gnulib developers.

Previous patch "[3/4] Use POSIX nm to simplify AIX export_symbols_cmds."
now is "[2/3] support POSIX nm, prefer POSIX nm for AIX", and addresses
Peter's concerns in above discussions:
* respect the user provided NM value
* work with (user provided) NM (even without BSD- or POSIX-flag)
* more strictly check for nm's POSIX interface
* emit a warning when the user provided NM fails to mark weak symbols

And finally, patch "AIX: Stop exporting any _GLOBAL__ symbol." still
applies for more robust g++ exception handling.

Indeed the patches feel more straightforward now.

Thanks for reconsideration!
/haubi/