Default two more of the options from env variables.

For build systems that don't allow someone to easily add in generator args,
support env variables for the values.

Most the bool env var support to a helper since it is used in three places now.

src/google/protobuf/compiler/objectivec/objectivec_helpers.cc

@@ -73,6 +73,14 @@ using ::open;
 
 namespace {
 
+bool BoolFromEnvVar(const char* env_var, bool default_value) {
+  const char* value = getenv(env_var);
+  if (value) {
+    return std::string("YES") == ToUpper(value);
+  }
+  return default_value;
+}
+
 class SimpleLineCollector : public LineConsumer {
  public:
   SimpleLineCollector(std::unordered_set<std::string>* inout_set)
@@ -117,9 +125,7 @@ PrefixModeStorage::PrefixModeStorage() {
   // Even thought there are generation options, have an env back door since some
   // of these helpers could be used in other plugins.
 
-  const char* use_package_cstr = getenv("GPB_OBJC_USE_PACKAGE_AS_PREFIX");
-  use_package_name_ =
-    (use_package_cstr && (std::string("YES") == ToUpper(use_package_cstr)));
+  use_package_name_ = BoolFromEnvVar("GPB_OBJC_USE_PACKAGE_AS_PREFIX", false);
 
   const char* exception_path = getenv("GPB_OBJC_PACKAGE_PREFIX_EXCEPTIONS_PATH");
   if (exception_path) {
@@ -180,7 +186,9 @@ void SetProtoPackagePrefixExceptionList(const std::string& file_path) {
 }
 
 Options::Options() {
-  // Default is the value of the env for the package prefixes.
+  // While there are generator options, also support env variables to help with
+  // build systems where it isn't as easy to hook in for add the generation
+  // options when invoking protoc.
   const char* file_path = getenv("GPB_OBJC_EXPECTED_PACKAGE_PREFIXES");
   if (file_path) {
     expected_prefixes_path = file_path;
@@ -190,8 +198,9 @@ Options::Options() {
     expected_prefixes_suppressions =
         Split(suppressions, ";", true);
   }
-  prefixes_must_be_registered = false;
-  require_prefixes = false;
+  prefixes_must_be_registered =
+      BoolFromEnvVar("GPB_OBJC_PREFIXES_MUST_BE_REGISTERED", false);
+  require_prefixes = BoolFromEnvVar("GPB_OBJC_REQUIRE_PREFIXES", false);
 }
 
 namespace {