The upstream configure script doesn't currently seem to work this way
for any of the ARM extensions.  Instead, you seem need to build with the
right --extra-cflags as well as enabling the extension(s) you want, such
as --enable-armvfp or --enable-neon.

The configure script has the facilities to add cflags in this way
though; it's used for the ultraspac vis extension (whatever that is);
but I don't know what the upstream community prefers.

In addition to the above, it looks like -march=armv7-a should also be
added to extra-cflags when building the NEON flavour; otherwise the
ffmpeg configuration assumes that the PLD (cache preload) instruction
can't be used because the baseline build architecture is armv5.  But it
doesn't really make sense to assume that any ARMv7 features may be
absent if NEON is present.

This would give --extra-cflags='-march=armv7-a -mfpu=neon -mfloat-
abi=softfp' for the neon flavour.

I'll point Måns at this thread and ask for his comments.

-- 
Integrate and enable ARMv5TE/v6/VFP and NEON optimisations from ffmpeg trunk 
for armel
https://bugs.launchpad.net/bugs/383240
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