-Eliminate the need for the porttime.h header by PortMidiMusicSystem. The only PortTime function in use was Pt_Sleep, which is a wrapper for usleep/Sleep and is replaced with I_Sleep.

git-svn-id: http://odamex.net/svn/root/trunk@4461 b0bcef46-2027-0410-bd56-ab726e9a1d04
-Change the CMake PortMidi module to not search for the PortTime library. This search is only done on non-Win32 platforms, where it expects a pre-existing PortMidi library.

git-svn-id: http://odamex.net/svn/root/trunk@4462 b0bcef46-2027-0410-bd56-ab726e9a1d04