Remove automatically adding the /MP flag for msvc build
Remove automatically adding the /MP flag for msvc build

Using VS 2013, testing the flag is now causing CMake to hang during
the configure phase. Seems likely that a recent Microsoft update
is causing this since it hadn't been an issue. Regardless, it
generally isn't a good idea to automatically add this flag.

linus-sherrill/maptk,linus-sherrill/maptk,linus-sherrill/maptk,linus-sherrill/maptk,linus-sherrill/maptk