Use "MultiProcessorCompilation" instead of explicit /MP in MSVS projects.

Use the user-visible option instead of the command line option which is now
hidden in the IDE.

Closes #16093.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76157 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775