on Windows, byteswap.h does not exist, but gcc builtin bswap* does
Forget my comment, something else:
The code seems to be able to check for the buildin anyways in the code. So we could simply do EINA_HAS_BUILTIN(__builtin_bswap32) in eina_cpu_inline.x and do no check at all in meson ? What do you think about that ? That would solve our problem in that regard. and simplify the buildsystem.