For sys/dev/drm2/radeon, only use -fms-extensions with gcc. This flag
is only to stop gcc complaining about anonymous unions, which clang does
not do. For clang 3.4 however, -fms-extensions enables the Microsoft
__wchar_t type, which clashes with our own types.h.
MFC r260102:
Similar to r260020, only use -fms-extensions with gcc, for all other
modules which require this flag to compile. Use a GCC_MS_EXTENSIONS
variable, defined in kern.pre.mk, which can be used to easily supply the
flag (or not), depending on the compiler type.
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