Caused by f04260d8c6.
Cycles' CMake defines macros with the same name as Blender, which
override the Blender ones. There's however a small difference in the
re-defined `remove_cc_flag()`, the Cycles version only takes one flag at
a time. So I guess Blender's calls to it would only result in the first
flag being removed.
Of course Cycles shouldn't override any Blender macros, but I'll leave
that up to Brecht to fix properly.
e73c60f1bd