Restore the libcrypto C++ runtime dependency, but with a build flag

This almost stuck but broke some targets in Android. So we can work on
clearing out those issues, restore the vtable, but gate it on a build
flag, so Android can easily turn it back off. We'll then gradually fix
Android until the build flag can be removed.

Update-Note: If the build fails due to vtables, build with
`BORINGSSL_TEMPORARY_NO_CXX_RUNTIME` and then contact the BoringSSL
team, so we can help fix your build. It is expected that only Android
will need to do this.

Bug: 486922845
Change-Id: Iec1548f7b503bd561836b261c5ca0d2b7acdf43a
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/96467
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
Auto-Submit: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

crypto/evp/internal.h

diff --git a/crypto/evp/internal.h b/crypto/evp/internal.h
index d843933..6692c58 100644
--- a/crypto/evp/internal.h
+++ b/crypto/evp/internal.h
@@ -229,10 +229,20 @@
  public:
   static constexpr bool kAllowUniquePtr = true;
 
-  // TODO(crbug.com/487376811): Ideally this destructor should be virtual so
-  // that we can emit vtables in libcrypto. In that case we would be able to
-  // replace `pmeth` with virtual methods and subclassing.
+  // Use an optional virtual destructor. This class does not (yet) have a need
+  // for a vtable, but we intend to write code with vtables in the future.
+  // Virtual destructors add a reference to symbols in the C++ runtime, so trial
+  // the dependency here. This is, temporarily, gated on a build define. If this
+  // breaks your build, build with `BORINGSSL_TEMPORARY_NO_CXX_RUNTIME` and then
+  // contact the BoringSSL team, so we can help fix your build.
+  //
+  // TODO(crbug.com/486922845): Remove the `BORINGSSL_TEMPORARY_NO_CXX_RUNTIME`
+  // case.
+#if defined(BORINGSSL_TEMPORARY_NO_CXX_RUNTIME)
   ~EvpPkeyCtx();
+#else
+  virtual ~EvpPkeyCtx();
+#endif
 
   // Method associated with this operation
   const bssl::EVP_PKEY_CTX_METHOD *pmeth = nullptr;