Add a check for libexecinfo on FreeBSD.
Add a check for libexecinfo on FreeBSD.

Interestingly enough, when compiling with clang, it picks up that
libexecinfo is needed for backtrace (or perhaps it has a builtin).
But with gcc, linking fails.

configure.ac:
	Add a check for backtrace in libexecinfo, so that
	libexecinfo gets added to the build.

LinearTapeFileSystem/ltfs,LinearTapeFileSystem/ltfs,LinearTapeFileSystem/ltfs,LinearTapeFileSystem/ltfs,LinearTapeFileSystem/ltfs