Internal users often have two builds for `boring`, one using a
precompiled build of boringSSL and another built from source with
patches applied. However the features that enable these builds are
mutually exclusive. For example, the `"pq-experimental"` feature is
required to build the source with all of the necessary codepoints for PQ
key exchange, but if this feature is enabled and a precompiled boringSSL
is provided, then the build will fail. This means users will have to
also control their builds with mutually exclusive features.
An alternative is to *ignore* features that enable patches whenever a
precompiled boringSSL is provided. This is a little different from the
"assume patched" environment variable, which applies whenever we're
building from source.