Add ENABLE_PYPROTO_API option to CMake build, clean up compilation

The PYBIND11_PROTOBUF_ENABLE_PYPROTO_API define was never set in
CMake builds, add a corresponding option.

As the check_unknown_fields code is only called dependent on the
define remove it from the build if not used.

CMakeLists.txt

@@ -17,6 +17,7 @@ set(CMAKE_POSITION_INDEPENDENT_CODE ON)
 # Options
 
 option(BUILD_TESTS "Build tests." OFF)
+option(ENABLE_PYPROTO_API "Enable usage of proto_api." OFF)
 
 # ============================================================================
 # Find Python
@@ -69,9 +70,20 @@ add_library(
   pybind11_protobuf/proto_cast_util.cc
   pybind11_protobuf/proto_cast_util.h
   pybind11_protobuf/proto_caster_impl.h
-  # bazel: cc_library::check_unknown_fields
-  pybind11_protobuf/check_unknown_fields.cc
-  pybind11_protobuf/check_unknown_fields.h)
+)
+
+if(ENABLE_PYPROTO_API)
+  target_sources(
+    pybind11_native_proto_caster
+    PRIVATE
+      # bazel: cc_library::check_unknown_fields
+      pybind11_protobuf/check_unknown_fields.cc
+      pybind11_protobuf/check_unknown_fields.h
+  )
+  target_compile_definitions(pybind11_native_proto_caster
+    PRIVATE PYBIND11_PROTOBUF_ENABLE_PYPROTO_API
+  )
+endif()
 
 target_link_libraries(
   pybind11_native_proto_caster
@@ -99,9 +111,20 @@ add_library(
   pybind11_protobuf/proto_cast_util.cc
   pybind11_protobuf/proto_cast_util.h
   pybind11_protobuf/proto_caster_impl.h
-  # bazel: cc_library: check_unknown_fields
-  pybind11_protobuf/check_unknown_fields.cc
-  pybind11_protobuf/check_unknown_fields.h)
+)
+
+if(ENABLE_PYPROTO_API)
+  target_sources(
+    pybind11_wrapped_proto_caster
+    PRIVATE
+      # bazel: cc_library::check_unknown_fields
+      pybind11_protobuf/check_unknown_fields.cc
+      pybind11_protobuf/check_unknown_fields.h
+  )
+  target_compile_definitions(pybind11_wrapped_proto_caster
+    PRIVATE PYBIND11_PROTOBUF_ENABLE_PYPROTO_API
+  )
+endif()
 
 target_link_libraries(
   pybind11_wrapped_proto_caster
@@ -119,9 +142,6 @@ target_include_directories(
   PRIVATE ${PROJECT_SOURCE_DIR} ${protobuf_INCLUDE_DIRS} ${protobuf_SOURCE_DIR}
           ${pybind11_INCLUDE_DIRS})
 
-# TODO set defines PYBIND11_PROTOBUF_ENABLE_PYPROTO_API see: bazel:
-# pybind_library: proto_cast_util
-
 # bazel equivs. checklist
 #
 # bazel: pybind_library: enum_type_caster - enum_type_caster.h