crypto/bio/bio_test.cc - I'm not sure where this was added for, but none
   of the functions used there appear to have feature macros documented.
crypto/bio/printf.c - -std=c99 provides (v)snprintf.
crypto/lhash/lhash_test.cc - we no longer call rand_r.
crypto/mem.c - we no longer call strdup and -std=c99 provides (v)snprintf.

Apple messed up their headers and, if _POSIX_C_SOURCE is defined but
_DARWIN_C_SOURCE isn't, pthread.h no longer defines mach_port_t. They
then shipped a version of libc++ headers that is missing this fix, so
the build breaks:
bcc92d75df

If one uses XCode, they've hacked their pthread.h to provide mach_port_t
if defined(__cplusplus), but the standalone tools appear to be old and
missing this.

We can work around this by also defining _DARWIN_C_SOURCE in C++ files
that need _POSIX_C_SOURCE, but it appears none of these files actually
need it.

Change-Id: I5df9453730696100eb22b809febeb65053701322
Reviewed-on: https://boringssl-review.googlesource.com/20964
Reviewed-by: Adam Langley <agl@google.com>