vchoi_fork

[svn-r26412] Merge of r26081-2, 26226, and 26399 from the autotools rework branch.

Various tweaks for autotools thread-safety and Pthreads support.

- Moved the check for pthread_attr_setscope() into the thread-safe
  checks section. Documented its necessity and added a cross-compiling
  option and helpful comment.

- Moved the high-level library checks up to the same place where
  Fortran and C++ are checked. This will make it easier to handle
  threadsafe/high-level combinations later.

- Also changed the default of --enable-pthread to 'check', which is
  the same as the old 'yes' behavior where we just check the standard
  locations. 'yes' and 'no' are still accepted, though 'no' will
  currently produce an error since the autotools only support Pthreads.

Fixes: HDFFV-9087

Tested on: h5committest
           jam (w/ threadsafe)