--enable-threadsafe no longer requires --with-pthreads if Pthreads can
be found in the standard include and library paths.

Tested on:
    32-bit LE linux (jam)
    SunOS 5.11 (emu)
    64-bit Darwin 12.5 (kite)
    64-bit FreeBSD 8.2 (freedom)

Testing focused on making sure that Pthreads could be found on different
platforms and investigating the interaction of Fortran/C++/thread-safe
options.


git-svn-id: https://svn.hdfgroup.uiuc.edu/hdf5/trunk@24857 dab4d1a6-ed17-0410-a064-d1ae371a2980