Document that disable-static plus -all-static is not portable.

Document that disable-static plus -all-static is not portable.
* doc/libtool.texi (Link mode): -all-static does not go well
with disable-static on most systems.
* tests/static.at (static linking flags for programs) [!aix]:
With --disable-static but nonempty link_static_flag, skip the
m-all-static link.  Do not skip it on AIX, as there the
enable_static is self-inflicted, and thus expected to work.
* THANKS: Update.
Report by Nix.