Building engine on macOS

[adriangonz]

Currently, the engine cannot be built in macOS as-is. In particular, the problem seems to lie with this line in the proto/k8s/Makefile.

seldon-core/proto/k8s/Makefile

Line 5 in ed63ed7

sed --in-place 's|import "k8s.io/apiextensions-apiserver/|//import "k8s.io/apiextensions-apiserver/|' k8s.io/api/core/v1/generated.proto

The --in-place flag in sed doesn't work in the BSD version. Since macOS is based on BSD, the build fails at that step. Instead, we could use -i <extension> which works both on the GNU and BSD versions of sed.