Replace global configuration of FFmpeg provisioning with parameter

FFmpeg provisioning script for Windows loads common configuration options from a file and stores them in a global variable which is modified per toolset (LLVM, MinGW, MSVC, Android). This results in possible side effects when an installation for a certain toolset is appending to the current state of the configuration.

Better approach would be to replace global variable usage with additional parameter to InstallFfmpeg function and make each specific installation pass complete set of configuration options that it requires.