libtool: speed up ltwrapper_script detection in execute mode

Execute mode is slow and might even DOS the computer in extreme
cases when a parameter is a big binary file without newlines.
Work around this with different truncation if a suitable dd
utility is found.

Fixes bug#13472 and bug#16662.

Reported by Pavel Raiskup and Nick Bowler.

* m4/libtool.m4 (_LT_PATH_DD): New macro, for finding a dd utility
that works for the below purpose.
(_LT_CMD_TRUNCATE): New macro, for finding out how to truncate binary
pipes (fallback to the old sed truncation if no suitable dd is found
in _LT_PATH_DD).
(_LT_SETUP): Require _LT_CMD_TRUNCATE.
(LT_INIT): Require Autoconf 2.62, as needed by _LT_PATH_DD.
* build_aux/ltmain.in (func_lalib_p): Factor out the actual "generated
by libtool" test into...
(func_generated_by_libtool_p): ...this new function...
(func_ltwrapper_script_p): ...so that it can be reused here, when
truncating the pipe according to _LT_CMD_TRUNCATE.
* THANKS: Update.

Signed-off-by: Peter Rosin <peda@lysator.liu.se>
libtool: fix nm test for MSYS/MinGW

The check for the -B option of nm does not work as intended on MSYS/MinGW.
MSYS converts /dev/null to the DOW/Windows "equivanent" special file NUL,
but the MinGW nm treats this file as any empty file. This means that
you might end up with some fallback nm instead of the desired nm. This
is not normally a problem, but if one nm is built without lto support, it
starts to matter.

Fixes sr #108558, reported by LRN.

* m4/libtool.m4 (LT_PATH_NM) [MSYS]: Use a non-existant file instead of
/dev/null when checking if nm supports -B.

Signed-off-by: Peter Rosin <peda@lysator.liu.se>
libtool: delay expansion of $ECHO until the wrapper script runs

Expanding $ECHO too early leads to unquoted \n when ECHO='printf %s\n'.

Fixes bug#17478. Reported by Egmont Koblinger.

* build-aux/ltmain.in (func_emit_wrapper) [fast_install]: Quote the
$ECHO, and delay the expansion.

Signed-off-by: Peter Rosin <peda@lysator.liu.se>