This adds a feature to build against a BoringSSL version compatible with
the current boringssl-fips, but _without_ actually enabling the `fips`
feature.
This can be useful to use with `fips-link-precompiled` while using a
custom BoringSSL version based on the older FIPS branch.
This feature expects a recent boringssl checkout (such as the one
found in boring-sys/deps/boringssl), so it should not be using
the same bindings as the fips feature, which are based on
boring-sys/deps/boringssl-fips, which is older and with a different
API.
Kornel