vchoi_fork

[svn-r26423] Made the thread-safe + high-level library combination "unsupported".

A detailed error message is emitted if this combination is attempted.
Builders can use --enable-unsupported to build the combo.

Note that default builds will now need to use --disable-hl with
--enable-threadsafe since the high-level library is built by default. 

Part of: HDFFV-8719

Tested on: Local Linux w/ various combinations of configure options.