CMake: Make sure we define flags variables for Distribution

This fixes a CMake error with the MSVC generator, but it's also
generally good to have around.

dtool/CompilerFlags.cmake

@@ -155,3 +155,11 @@ if(NOT MSVC)
     add_compile_options("-fvisibility=hidden")
   endif()
 endif()
+
+# These are flags for the custom configurations we add
+# Distribution
+set(CMAKE_C_FLAGS_DISTRIBUTION "")
+set(CMAKE_CXX_FLAGS_DISTRIBUTION "")
+set(CMAKE_SHARED_LINKER_FLAGS_DISTRIBUTION "")
+set(CMAKE_MODULE_LINKER_FLAGS_DISTRIBUTION "")
+set(CMAKE_EXE_LINKER_FLAGS_DISTRIBUTION "")