fix fenv for Windows

Please comment on the correct attribute for FE_DFL_ENV.
In C it's a macro.
I see _Fenv0 is marked "__declspec(selectany)" in MSVCR12, is this the same in older versions? If not, there might be linking issues (with the statically linked C runtime library) .

The symbol does not exist in the import library for msvcrt*.lib, so "private extern __gshared fenv_t _Fenv0;"  would be no good.