Log Message:
As the seemingly untested -Werror changes break the bootstrap at least
on Linux, update libnbcompat to 20150615:
- Assume a C89 environment and malloc, vprintf, memcpy, memset, memmove
  to be present and working.
- Use full prototypes for isblank, snprintf, asprintf and strdup.
  This avoids GCC failing to do conflicts with the builtins.