[FR] CMake: add prefix "SRT" to all srt cmake options

[IvanPysmenni]

There is an inconvenience with configuration srt lib build in a case when srt was added to CMake project as subdirectory. SRT options doesn't have an unique name. So during configuration in root CMake file there is a chance that this name is already used by another project in your build(for example ENABLE_APPS). It would be to add prefix "SRT" to all srt cmake options. For example: SRT_ENABLE_APPS.

As an example this is how it is done in spdlog:
https://github.com/gabime/spdlog/blob/v1.x/CMakeLists.txt#L59