Add clarification to download location and extlibs.

git-svn-id: https://svn.hdfgroup.uiuc.edu/hdf5/trunk@25130 dab4d1a6-ed17-0410-a064-d1ae371a2980
Rudimentary changes; changed the casting of hid_t_f variables that used a casting of int_f to use a casting of hid_t_f.

Tested: jam (gnu) 





git-svn-id: https://svn.hdfgroup.uiuc.edu/hdf5/trunk@25133 dab4d1a6-ed17-0410-a064-d1ae371a2980