Add autoconf macro to check for the bswap16 intrinsic.

Signed-off-by: Erik de Castro Lopo <erikd@mega-nerd.com>
Use a configure check for bswap16 instead of gcc version #ifdefs.

Fixes a build problem on apple clang.

Signed-off-by: Erik de Castro Lopo <erikd@mega-nerd.com>
Clean up bswap32 autoconf macro.

Improve 'checking' message and don't hog the cache namespace.

Signed-off-by: Erik de Castro Lopo <erikd@mega-nerd.com>
Fix indenting on the bswap32 autoconf macro.

We generally prefer spaces to tabs.

Signed-off-by: Erik de Castro Lopo <erikd@mega-nerd.com>