libobs: avoid using to w32-pthreads for mingw-w64

mingw-w64 provides its own pthreads library.

also change some link libs as mingw-w64 libraries are not postfixed .lib

libobs/CMakeLists.txt

@@ -46,9 +46,12 @@ if(WIN32)
 		util/threading-windows.c
 		util/pipe-windows.c
 		util/platform-windows.c)
-	set(libobs_PLATFORM_DEPS
-		w32-pthreads
-		winmm.lib)
+	set(libobs_PLATFORM_DEPS winmm)
+	if(MSVC)
+		set(libobs_PLATFORM_DEPS
+		${libobs_PLATFORM_DEPS}
+		w32-pthreads)
+	endif()
 elseif(APPLE)
 	set(libobs_PLATFORM_SOURCES
 		obs-cocoa.c