Require source files define __STDC_FORMAT_MACROS to use BN FMT macros.

inttypes.h kindly requires a feature macro in C++ on some platforms, due
to a bizarre footnote in C99 (see footnote 191 in section 7.8.1). As
bn.h is a public header, we must leak this wart to the consumer. On
platforms with unfriendly inttypes.h headers, using BN_DEC_FMT1 and
friends now require the feature macro be defined externally.

This broke the Chromium Android Clang builder:
http://build.chromium.org/p/chromium.linux/builders/Android%20Clang%20Builder%20%28dbg%29/builds/59288

Change-Id: I88275a6788c7babd0eae32cae86f115bfa93a591
Reviewed-on: https://boringssl-review.googlesource.com/4688
Reviewed-by: Adam Langley <agl@google.com>