Sync up error.c with gnulib

Summary of changes:

- Use of !_LIBC instead of HAVE_CONFIG_H
- Code changes in [!_LIBC] that don't affect us
- Minor formatting changes
- Use __builtin_expect in shared code
- Define some macros in [_LIBC] that are used in gnulib but never
  defined in glibc
- Flip macro check for STRERROR_R_CHAR_P so that it does not throw a
  warning