In some cases, consumers may include a BoringSSL header without setting
up include paths. This risks pulling in system OpenSSL headers instead.
For almost every BoringSSL header, the first #include is base.h, which
does not exist upstream, thus the mistake will be caught.
The exception is base.h itself which naturally does not include itself.
Have it include an empty is_boringssl.h header to catch this mistake.
Change-Id: Ia96586ecc627ff46867d8af8b68138185866f074
Reviewed-on: https://boringssl-review.googlesource.com/14949
Reviewed-by: Adam Langley <agl@google.com>
