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
<=gcc-5 with -Wold-style-cast warn when expansions of SDL macro
CD_INDRIVE produce an old style cast, even though the cast comes from a
system header. Undefine it and define an equivalent that uses a C++
cast, so that uses do not provoke a warning.