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.
MFC r260322:
In addition to r260102, also define GCC_MS_EXTENSIONS in bsd.sys.mk,
since kernel module builds do not use kern.pre.mk.