libtool.m4: Avoid a broken AC_TRY_EVAL macro

libtool.m4: Avoid a broken AC_TRY_EVAL macro

As said in the Autoconf source, the AC_TRY_EVAL macro is dangerous and
undocumented, and should not be used.
In particular, the one related to nm yields binary data in the config.log
file with dash, where "echo \\1" (echo with the argument \1) produces the
control character ^A instead of the usual \1 with most shells (POSIX says
that the result is implementation-defined). See:
  https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=910076
This patch attempts to replace this AC_TRY_EVAL occurrence by code with
similar behavior, but using $ECHO instead of echo in order to avoid the
backslash issue.

https://debbugs.gnu.org/cgi/bugreport.cgi?bug=21309