util: Cope with LONG_BIT not being defined on Windows.

util: Cope with LONG_BIT not being defined on Windows.

Neither MSVC nor MinGW defines LONG_BIT.  For MSVC this was not a problem as
it doesn't define __x86_64__ macro (it's GCC specific.)

However on Windows long type is guaranteed to be 32bits.

Also add an #error, as GCC will just warn, not throw any error, when no
value is returned.

Trivial.