Enable Watcom WMake generator on Darwin

Open Watcom v2 runs on Mac OS and is useful there for doing
cross-compilation.

Source/CMakeLists.txt

@@ -803,7 +803,7 @@ if (WIN32)
 endif ()
 
 # Watcom support
-if(WIN32 OR CMAKE_SYSTEM_NAME STREQUAL "Linux")
+if(WIN32 OR CMAKE_SYSTEM_NAME STREQUAL "Linux" OR CMAKE_SYSTEM_NAME STREQUAL "Darwin")
   set_property(SOURCE cmake.cxx APPEND PROPERTY COMPILE_DEFINITIONS CMAKE_USE_WMAKE)
   list(APPEND SRCS
     cmGlobalWatcomWMakeGenerator.cxx