mingw-w64 fixes (#317)
mingw-w64 fixes (#317)

* cmake: use FindThreads module
* cmake: don't use msvc syntax on pkg-config file
* cmake: include ws2_32 for mingw too

Cinegy/srt,Haivision/srt,Cinegy/srt,ethouris/srt,Cinegy/srt,ethouris/srt,Haivision/srt,ethouris/srt,Cinegy/srt,ethouris/srt,Haivision/srt,Haivision/srt