cmake: make DEPENDENCIES in protobuf_generate() a multi-value keyword

[71]

It was previously a one-value keyword, so given

protobuf_generate(
  ...
  DEPENDENCIES
    a
    b
)

a would be a dependency, and b would be discarded (or, more precisely, added to protobuf_generate_UNPARSED_ARGUMENTS).

This can be confirmed with the following changes:

diff --git a/cmake/protobuf-generate.cmake b/cmake/protobuf-generate.cmake
index 244407ee2..6dfbcef6d 100644
--- a/cmake/protobuf-generate.cmake
+++ b/cmake/protobuf-generate.cmake
@@ -10,6 +10,9 @@ function(protobuf_generate)

   cmake_parse_arguments(protobuf_generate "${_options}" "${_singleargs}" "${_multiargs}" "${ARGN}")

+  message("Deps: ${protobuf_generate_DEPENDENCIES}")
+  message("Unparsed args: ${protobuf_generate_UNPARSED_ARGUMENTS}")
+
   if(NOT protobuf_generate_PROTOS AND NOT protobuf_generate_TARGET)
     message(SEND_ERROR "Error: protobuf_generate called without any targets or source files")
     return()
diff --git a/cmake/tests.cmake b/cmake/tests.cmake
index 7976546fe..1cadf1e5a 100644
--- a/cmake/tests.cmake
+++ b/cmake/tests.cmake
@@ -27,6 +27,7 @@ foreach(proto_file ${lite_test_protos})
     LANGUAGE cpp
     OUT_VAR pb_generated_files
     IMPORT_DIRS ${protobuf_SOURCE_DIR}/src
+    DEPENDENCIES a b
   )
   set(lite_test_proto_files ${lite_test_proto_files} ${pb_generated_files})
 endforeach(proto_file)

Which printed "Deps: a" and "Unparsed args: b" for lite test files. With this change, it now prints "Deps: a;b" and "Unparsed args: ".

AFAIK, making DEPENDENCIES a multi-value keyword is mostly backwards-compatible; however, users who specified dependencies that were being ignored may be surprised to see that they are no longer ignored (which could cause build errors).

To prevent this kind of problem in the future, validating that protobuf_generate_UNPARSED_ARGUMENTS is empty could be useful, but to avoid potentially breaking more builds I did not do this in this PR.