preproc: Use the same format as #error for reporting #warning lines

Adds a preprocessor_streambuf::warning() method, equivalent to
preprocessor_streambuf::error() but logging to the warning logger and
returning to the caller instead of throwing an exception.

changelog

@@ -48,6 +48,8 @@ Version 1.12.0+dev:
      honored by Windows builds using Boost.filesystem (bug #22967).
    * Fixed dotfiles being included when {including} directories in WML, on
      builds using the new filesystem code (regression introduced in 1.11.19).
+   * Preprocessor #warning messages now conform better to the new WML
+     parser/preprocessor diagnostics format introduced in version 1.11.10.
  * Miscellaneous and bug fixes:
    * Fixed non-ASCII characters in the user's path prevent
      BfW to launch on windows (bug #22983)

src/serialization/preprocessor.cpp

@@ -240,6 +240,7 @@ class preprocessor_streambuf: public streambuf
 public:
 	preprocessor_streambuf(preproc_map *);
 	void error(const std::string &, int);
+	void warning(const std::string &, int);
 };
 
 preprocessor_streambuf::preprocessor_streambuf(preproc_map *def) :
@@ -357,6 +358,17 @@ void preprocessor_streambuf::error(const std::string& error_type, int l)
 	throw preproc_config::error(error);
 }
 
+void preprocessor_streambuf::warning(const std::string& warning_type, int l)
+{
+	std::string position, warning;
+	std::ostringstream pos;
+	pos << l << ' ' << location_;
+	position = lineno_string(pos.str());
+	warning = warning_type + '\n';
+	warning += "at " + position;
+	WRN_CF << warning << '\n';
+}
+
 
 /**
  * Sets up a new preprocessor for stream buffer \a t.
@@ -987,9 +999,9 @@ bool preprocessor_data::get_chunk()
 		} else if (command == "warning") {
 			if (!skipping_) {
 				skip_spaces();
-				std::string message = read_rest_of_line();
-				WRN_CF << "#warning: \"" << message << "\" at "
-					<< linenum_ << ' ' << get_location(target_.location_) << '\n';
+				std::ostringstream warning;
+				warning << "#warning: \"" << read_rest_of_line() << '"';
+				target_.warning(warning.str(), linenum_);
 			} else
 				DBG_CF << "Skipped a warning\n";
 		} else