BUILD: add flag for building protoc plug-in

Author: kamahen

CMakeLists.txt

@@ -111,11 +111,17 @@ option(BUILD_TESTING
 option(SKIP_SSL_TESTS
        "Skip the SSL tests"
        OFF)
-# The following test requires installing protobuf-compiler python3-protobuf
-# libprotobuf-devel (or protobuf-devel)
+# The following test requires installing packages protobuf-compiler
+# python3-protobuf libprotobuf-devel (or protobuf-devel):
 option(TEST_PROTOBUFS_PROTOC
        "Run protobuf tests that require protoc, etc."
        OFF)
+# The following only works on Linux, and downloads some files from
+# github. It may cause protobufs:bootstrap test to fail and may
+# require some manual editing of files in packages/protobuf (q.v.):
+option(BUILD_PROTOBUFS_PROTOC
+       "Build protobuf with latest protoc"
+       OFF)
 option(BUILD_SWIPL_LD
        "Create the swipl-ld utility"
        ON)