scripts/build/osx: set CXX, too

In case go build will see a need to call C++ (rather than C)
compiler, CXX env var need to be properly set (to osxcross wrapper).

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

scripts/build/osx

@@ -11,6 +11,7 @@ export CGO_ENABLED=1
 export GOOS=darwin
 export GOARCH=amd64
 export CC=o64-clang
+export CXX=o64-clang++
 export LDFLAGS="$LDFLAGS -linkmode external -s"
 export LDFLAGS_STATIC_DOCKER='-extld='${CC}