CMake: Allow build system to provide protobuf compiler

When building without Hunter, it is desirable to pass the path to protoc and
the protobuf include directory from the build system. Allow these variables
to be overridden.

Fixes the error:

| CMake Error at cmake/functions.cmake:52 (message):
|   Protobuf_PROTOC_EXECUTABLE is empty
| Call Stack (most recent call first):
|   cmake/functions.cmake:96 (compile_proto_to_cpp)
|   src/crypto/protobuf/CMakeLists.txt:6 (add_proto_library)

Author: eigendude

cmake/functions.cmake

@@ -45,8 +45,13 @@ function(add_flag flag)
 endfunction()
 
 function(compile_proto_to_cpp PROTO_LIBRARY_NAME PB_H PB_CC PROTO)
-  get_target_property(Protobuf_INCLUDE_DIR protobuf::libprotobuf INTERFACE_INCLUDE_DIRECTORIES)
-  get_target_property(Protobuf_PROTOC_EXECUTABLE protobuf::protoc IMPORTED_LOCATION_RELEASE)
+  if (NOT Protobuf_INCLUDE_DIR)
+    get_target_property(Protobuf_INCLUDE_DIR protobuf::libprotobuf INTERFACE_INCLUDE_DIRECTORIES)
+  endif()
+  if (NOT Protobuf_PROTOC_EXECUTABLE)
+    get_target_property(Protobuf_PROTOC_EXECUTABLE protobuf::protoc IMPORTED_LOCATION_RELEASE)
+    set(PROTOBUF_DEPENDS protobuf::protoc)
+  endif()
 
   if (NOT Protobuf_PROTOC_EXECUTABLE)
     message(FATAL_ERROR "Protobuf_PROTOC_EXECUTABLE is empty")
@@ -78,7 +83,7 @@ function(compile_proto_to_cpp PROTO_LIBRARY_NAME PB_H PB_CC PROTO)
       COMMAND ${GEN_COMMAND}
       ARGS -I${PROJECT_SOURCE_DIR}/src -I${GEN_ARGS} --cpp_out=${SCHEMA_OUT_DIR} ${PROTO_ABS}
       WORKING_DIRECTORY ${CMAKE_BINARY_DIR}
-      DEPENDS protobuf::protoc
+      DEPENDS ${PROTOBUF_DEPENDS}
       VERBATIM
   )