For each link given as http://, verify that the site is accessible over
https:// and, if so, switch to it. These domains were converted:
* llvm.org
* clang.llvm.org
* en.cppreference.com
* www.dxx-rebirth.com
* www.libsdl.org
* www.scons.org
All users of physfs_list.h included fwd-partial_range.h or
partial_range.h; physfs_list.h did not include either, but assumed one
would be available. Include fwd to fix check_header_includes=1 build.
Fixes: ff67afd440 ("Propagate partial_range up into PHYSFSX_findFiles family")