Do not enable -Wold-style-cast for Windows

Bug: http://b/91353691

Enable -Wold-style-cast only for non-Windows targets.  _islower_l,
_isupper_l etc. in MinGW locale_win32.h (included from
libcxx/include/__locale) have an old-style-cast.

Test: Build and test Windows modules under Wine.
Change-Id: Ib7594559a43096885b0cc1c656cf59db8b52d38b
1 file changed