[tlbuild] PPC builds

[Karl Berry]

    Sounds like just adopting =check as an implicit default for
    libpng configure.

Libpng explicitly disrecommends this. From libpng-src/arm/arm_init.c:

#ifdef PNG_ARM_NEON_CHECK_SUPPORTED /* Do run-time checks */
/* WARNING: it is strongly recommended that you do not build libpng with
 * run-time checks for CPU features if at all possible.  In the case of the ARM
 * NEON instructions there is no processor-specific way of detecting the
 * presence of the required support, therefore run-time detection is extremely
 * OS specific.
[...]

Ditto VSX, as far as I can see.

    AFAICS, there is only one practical case where configuration of
    libpng's NEON code paths matter at all

At any rate, what worries me is involving the arm (or vsx) code on
non-arm (ppc) platforms. I didn't try it (maybe you'd like to), but my
guess is that it will break compilation, unless there is some kind of
checking that avoids it.

That's why I was asking for some kind of test program that can be
compiled (ideally just compiled, not run, so cross-compilation isn't
broken) by our libpng configure: if success, then enable arm-neon=check;
if failure, then don't. Wdyt?  --thanks, karl.