message: build: add ccache option to envoy build
  At least on my machine, ccache set via CC and CXX env variables is no
  longer working and breaks the boringssl build. This allows ccache to
  be turned on via cmake for the envoy code only.
diff: @@ -34,5 +34,14 @@ endif()
  option(ENVOY_STRIP "strip symbols from binaries" OFF)
  +option(ENVOY_USE_CCACHE "build with ccache" OFF)
  +if (ENVOY_USE_CCACHE)
  + find_program(CCACHE_FOUND ccache)
  + if (CCACHE_FOUND)
  + set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ccache)
  + set_property(GLOBAL PROPERTY RULE_LAUNCH_LINK ccache)
  + endif()
  +endif()
  +
  set(CMAKE_MODULE_PATH "${CMAKE_MODULE_PATH};${ENVOY_COTIRE_MODULE_DIR}")
  include(cotire)
  