Makefile: Have the Envoy compilation mode be configurable via ENVOY_COMPILATION_MODE
Makefile: Have the Envoy compilation mode be configurable via ENVOY_COMPILATION_MODE

Signed-off-by: Luke Shumaker <1abe3165bcc8c29e8b31e5c88636540fb9350004@datawire.io>

datawire/ambassador,datawire/ambassador,datawire/ambassador,datawire/ambassador,datawire/ambassador