Allow cross-compilation with protobuf

The upstream config module from protobuf always declares targets
for the protobuf library and protoc of the same platform. This means
one of the two is always wrong, as protoc needs to be run on the
build machine, but the library must be linked for the "host" platform.

A small change in the CMake scaffold allows the user to pass
`-DPROTOBUF_PROTOC_EXECUTABLE=<path/to/protoc>` to cmake when creating
the build tree.

Author: tobim

CMakeLists.txt

@@ -419,20 +419,21 @@ if(WITH_OTLP_GRPC
       endif()
     endif()
   endif()
-  # Latest Protobuf imported targets and without legacy module support
-  if(TARGET protobuf::protoc)
-    project_build_tools_get_imported_location(PROTOBUF_PROTOC_EXECUTABLE
-                                              protobuf::protoc)
-    # If protobuf::protoc is not a imported target, then we use the target
-    # directly for fallback
-    if(NOT PROTOBUF_PROTOC_EXECUTABLE)
-      set(PROTOBUF_PROTOC_EXECUTABLE protobuf::protoc)
+  if(NOT PROTOBUF_PROTOC_EXECUTABLE)
+    # Latest Protobuf imported targets and without legacy module support
+    if(TARGET protobuf::protoc)
+      project_build_tools_get_imported_location(PROTOBUF_PROTOC_EXECUTABLE
+                                                protobuf::protoc)
+      # If protobuf::protoc is not a imported target, then we use the target
+      # directly for fallback
+      if(NOT PROTOBUF_PROTOC_EXECUTABLE)
+        set(PROTOBUF_PROTOC_EXECUTABLE protobuf::protoc)
+      endif()
+    elseif(Protobuf_PROTOC_EXECUTABLE)
+      # Some versions of FindProtobuf.cmake uses mixed case instead of uppercase
+      set(PROTOBUF_PROTOC_EXECUTABLE ${Protobuf_PROTOC_EXECUTABLE})
     endif()
-  elseif(Protobuf_PROTOC_EXECUTABLE)
-    # Some versions of FindProtobuf.cmake uses mixed case instead of uppercase
-    set(PROTOBUF_PROTOC_EXECUTABLE ${Protobuf_PROTOC_EXECUTABLE})
   endif()
-  include(CMakeDependentOption)
 
   message(STATUS "PROTOBUF_PROTOC_EXECUTABLE=${PROTOBUF_PROTOC_EXECUTABLE}")
   set(SAVED_CMAKE_CXX_CLANG_TIDY ${CMAKE_CXX_CLANG_TIDY})