bug#72917: ffmpeg@{3,4,5} build failures on i686-linux

[Dariqq]

I was able to reconfigure the system on core updates by adding the 
following snippet into openals phases

#$@(if (target-x86-32?)
 #~((add-before 'configure 'unprotect
     (lambda* _
       (substitute* "CMakeLists.txt"
         (("if\\(HAVE_GCC_PROTECTED_VISIBILITY\\)") "if(0)")))))
 #~())
which disables the protection causing problems. I don't know what the 
implications of this change are.