fix safemalloc to compile w/o libbfd.

CMakeLists.txt:
  NOT_FOR_DISTRIBUTION option
cmake/readline.cmake:
  simplify libedit/readline detection.
  never use bundled libedit.
  use system readline v6 only if NOT_FOR_DISTRIBUTION=1
configure.cmake:
  use libbfd only if NOT_FOR_DISTRIBUTION=1
include/my_stacktrace.h:
  link with libbfd even w/o safemalloc.