ARM: disable use of system byteswap.h

Our bswap functions are better than those in glibc, which has
no ARM-optimised versions.

Originally committed as revision 14241 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -830,6 +830,9 @@ neon_deps="armv4l"
 ssse3_deps="x86"
 vis_deps="sparc"
 
+# system headers and functions
+byteswap_h_deps="!armv4l"
+
 # decoders / encoders
 ac3_decoder_deps="gpl !liba52"
 dxa_decoder_deps="zlib"