ExtIO wrapper for librtlsdr for use with HDSDR and other Winrad compatible programs

• Original code from jorgem-seq/ExtIO_RTL, which was derived from josemariaaraujo/ExtIO_RTL
• removed makefiles and added cmake support to compile with Visual Studio 2019
• added all required libraries as submodules
• added github action for automatic build
• merged 'Add Bias Tee option, add branch auto switch' from Jorge's testing branch

Clone with submodules to automatically retrieve libusb, pthread-win32 and librtlsdr from git bash:

git clone -b cmake_github_action --recursive https://github.com/hayguen/ExtIO_RTL.git

Manual compilation from Visual Studio 2019 Command Prompt:

cmake -G "Visual Studio 16 2019" -A Win32 -S ExtIO_RTL -B build_ExtIO_RTL
cmake --build build_ExtIO_RTL --config Release --target ExtIO_RTL

A pre-built binary should be available in github Actions.

GPL-2.0, see LICENSE file