Support old cmake for protobuf

Signed-off-by: owent <admin@owent.net>

CMakeLists.txt

@@ -128,7 +128,7 @@ if(WITH_OTLP)
   set(protobuf_MODULE_COMPATIBLE ON)
   find_package(Protobuf)
   find_package(gRPC)
-  if((NOT Protobuf_FOUND) OR (NOT gRPC_FOUND))
+  if((NOT Protobuf_FOUND AND NOT PROTOBUF_FOUND) OR (NOT gRPC_FOUND))
     if(WIN32 AND (NOT DEFINED CMAKE_TOOLCHAIN_FILE))
       install_windows_deps()
     endif()
@@ -140,7 +140,7 @@ if(WITH_OTLP)
       include(${CMAKE_TOOLCHAIN_FILE})
     endif()
 
-    if(NOT Protobuf_FOUND)
+    if(NOT Protobuf_FOUND AND NOT PROTOBUF_FOUND)
       find_package(Protobuf REQUIRED)
     endif()
     if(NOT gRPC_FOUND)
@@ -155,7 +155,9 @@ if(WITH_OTLP)
       endif()
     endif()
     # Latest Protobuf uses mixed case instead of uppercase
-    set(PROTOBUF_PROTOC_EXECUTABLE ${Protobuf_PROTOC_EXECUTABLE})
+    if(Protobuf_PROTOC_EXECUTABLE)
+      set(PROTOBUF_PROTOC_EXECUTABLE ${Protobuf_PROTOC_EXECUTABLE})
+    endif()
   endif()
   message("PROTOBUF_PROTOC_EXECUTABLE=${PROTOBUF_PROTOC_EXECUTABLE}")