arm: Macroize the test for 'setend' CPU instruction support

Signed-off-by: Diego Biurrun <diego@biurrun.de>
imgconvert: cosmetics: Reshuffle defines to reduce ifdeffery
http: Use a constant for the supported header size
libxvid: Drop PPC-specific CPU detection hack

It is doubtful if the hack (still) works and Xvid had ten years to fix it.
build: Add define for SIMD extensions requiring 16-byte aligned buffers
dct/rdft: Add missing typedefs for context structs

Without the typedefs there can be trouble depending on #include order.
fft-test: Drop unnecessary #ifdefs around header includes
fft-test: Comment some #endifs
configure: Extend the neon intrinsics check

Check for an instruction that causes the (very old) Apple GCC 4.2
to error out ("unrecognizable insn").

Signed-off-by: Martin Storsjö <martin@martin.st>