Allow ignoring macos package manager

Allow building without either homebrew or macports. This allows building in other environments, for example nixpkgs.

CMakeLists.txt

@@ -25,7 +25,7 @@ ENDIF()
 
 #shamelessly borrowed from FreeCAD project: https://github.com/FreeCAD/FreeCAD/blob/master/cMake/FreeCAD_Helpers/SetupPython.cmake
 # For building on OS X
-if(${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
+if(${CMAKE_SYSTEM_NAME} MATCHES "Darwin" AND NOT DEFINED IGNORE_PACKAGE_MANAGER)
 
   # If the user doesn't tell us which package manager they're using
   if(NOT DEFINED MACPORTS_PREFIX AND NOT DEFINED HOMEBREW_PREFIX)