cmake: Use multiprocessor compilation on Windows (#1605)

CMakeLists.txt

@@ -94,6 +94,7 @@ if(WIN32)
 endif()
 
 if(MSVC)
+	add_compile_options("/MP")
 	set(CMAKE_C_FLAGS_DEBUG "/DDEBUG=1 /D_DEBUG=1 ${CMAKE_C_FLAGS_DEBUG}")
 	set(CMAKE_CXX_FLAGS_DEBUG "/DDEBUG=1 /D_DEBUG=1 ${CMAKE_C_FLAGS_DEBUG}")