Hello,
I've been poking at gnash on and off, in hopes of having some way to view
Flash on Alpine.
So far, the problem that I know of is it insists on building a malloc that
supports mallinfo (which is used _very_ extensively in gnash).

This is jemalloc, where I've found 3 issues already:
First, it includes the header sys/sysctl.h by default; the #ifdefs around
this are plainly wrong and easily fixed.
Second, it defines issetugid() to 0; this of course breaks due to our
support for the function, and is horrendously wrong (though gnash is unlikely
to be configured in a way where it matters...).
Third, it uses some unsupported pthread stuff: I see
PTHREAD_ADAPTIVE_MUTEX_INITIALIZER_NP and PTHREAD_MUTEX_ADAPTIVE_NP.
At this point, it's beyond me.

Has anyone else looked at this?

I'm attaching my current WIP patches; less_sysctl.patch is probably correct,
but issetugid.diff is only to get it closer to building on musl and is
massively incomplete.
I hereby release both to the public domain, in case any work based on them
gets pushed upstream.

Thanks,
Isaac Dunham.