Use __builtin_ffs instead of ffs (#554)

Use __builtin_ffs instead of ffs (#554)

USE_BUILTIN_FFS is defined to 1 within __GNUC__, and the __builtin_ffs
function is available since GCC 3.x at least, while the ffs function
only exists on some OSes.

This fixes compilation for non-x86 mingw platforms. For x86,
USE_BUILTIN_FFS is explicitly disabled for windows targets - but
if USE_BUILTIN_FFS is enabled based on __GNUC__, it should also use
the builtin which actually is available correspondingly, not dependent
on the target OS.