-export-symbols for Linux

[Bruce Korb]

Sun is pretty much the same, though I accomplish it
slightly differently:

SUN:

> ( echo '{'
>   echo '  global:'
>   echo $extern_list | \
>     sed 's/\(.*\)/    \1;/'
>   echo '  local:'
>   echo '    *;'
>   echo '};'
> ) > $MODULE.map
> ${LD} -B reduce -M $MODULE.map -r -o $MODULE \
>   ${LDOPTS} ${OBJ} ${LIBS}

HP/UX, of course, is completely different.  In fact,
HP/UX-64 is completely different from HP/UX-32, too.