On assertion failure, print backtrace if available.

On assertion failure, print backtrace if available.

Merge from gnulib, incorporating:
2012-08-24 execinfo: port to FreeBSD
2012-08-22 execinfo: new module
* admin/merge-gnulib (GNULIB_MODULES): Add execinfo.
* lib/gnulib.mk, m4/gnulib-comp.m4: Regenerate.
* lib/execinfo.c, lib/execinfo.in.h, m4/execinfo.m4: New files.
* src/alloc.c [ENABLE_CHECKING]: Include <execinfo.h>.
(die) [ENABLE_CHECKING]: Print a backtrace if available.
* src/Makefile.in (LIB_EXECINFO): New macro.
(LIBES): Use it.