Add flag to enable verbose output

src/log.h

@@ -0,0 +1,40 @@
+#pragma once
+
+#include <iostream>
+#include <utility>
+
+// Forwards to an ostream, optionally
+class StreamProxy {
+public:
+	StreamProxy(std::ostream *os) : m_os(os) {}
+
+	template<typename T>
+	StreamProxy &operator<<(T &&arg)
+	{
+		if (m_os)
+			*m_os << std::forward<T>(arg);
+		return *this;
+	}
+
+	StreamProxy &operator<<(std::ostream &(*func)(std::ostream&))
+	{
+		if (m_os)
+			*m_os << func;
+		return *this;
+	}
+
+private:
+	std::ostream *m_os;
+};
+
+/// Error and warning output, forwards to std::cerr
+extern StreamProxy errorstream;
+/// Verbose output, might forward to std::cerr
+extern StreamProxy verbosestream;
+
+/**
+ * Configure log streams defined in this file.
+ * @param verbose enable verbose output
+ * @note not thread-safe!
+ */
+void configure_log_streams(bool verbose);