some newer binutils versions print scary warnings about protected data
because most gcc versions fail to produce the right address
references/relocations for such data that might be subject to copy
relocations. originally vis.h explicitly assigned default visibility
to all public data symbols to avoid this issue, but commit
b8dda24f removed this treatment for
stdin/out/err to work around a gcc 3.x bug, and since they don't
actually need it (because taking their addresses is not valid C).

instead, a check for the gcc 3.x bug is added to the configure check
for vis.h preinclude support; this feature will simply be disabled
when using a buggy version of gcc.