CMake: Enable multi-processor compilation for MSVC

Add option wxBUILD_MSVC_MULTIPROC which defaults to ON
to add /MP the MSVC compiler options.

build/cmake/init.cmake

@@ -22,6 +22,11 @@ if(DEFINED wxBUILD_USE_STATIC_RUNTIME AND wxBUILD_USE_STATIC_RUNTIME)
     endforeach()
 endif()
 
+if(wxBUILD_MSVC_MULTIPROC)
+    wx_string_append(CMAKE_C_FLAGS " /MP")
+    wx_string_append(CMAKE_CXX_FLAGS " /MP")
+endif()
+
 if(wxBUILD_COMPATIBILITY VERSION_LESS 3.0)
     set(WXWIN_COMPATIBILITY_2_8 ON)
 endif()

build/cmake/options.cmake

@@ -25,6 +25,7 @@ mark_as_advanced(wxBUILD_CUSTOM_SETUP_HEADER_PATH)
 
 if(MSVC)
     wx_option(wxBUILD_USE_STATIC_RUNTIME "Link using the static runtime library" OFF)
+    wx_option(wxBUILD_MSVC_MULTIPROC "Enable multi-processor compilation for MSVC")
 else()
     # Other compilers support setting the C++ standard, present it an option to the user
     if(DEFINED CMAKE_CXX_STANDARD)