Improve MSVC 2015 compatibility of FileSource plugin (fixes #348)

SoapySDR: enhanced log message during scan to show which id parameters are used
Windows build fixes
Perseus server: fixed missing libusb libraries in cmake file

plugins/samplesource/filesource/filesourceinput.cpp

@@ -181,7 +181,7 @@ bool FileSourceInput::start()
 	QMutexLocker mutexLocker(&m_mutex);
 	qDebug() << "FileSourceInput::start";
 
-	if (m_ifstream.tellg() != 0) {
+	if (m_ifstream.tellg() != (std::streampos)0) {
 		m_ifstream.clear();
 		m_ifstream.seekg(sizeof(FileRecord::Header), std::ios::beg);
 	}