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
Per C99, this code was always wrong. The results of `snprintf` are
undefined if the target string array and one of the supplied input
arguments refer to the same buffer. In practice, this particular usage
happened to work by wastefully copying a string onto itself, then adding
more content at the end. Switch to appending properly, which also fixes
the gcc-8 -Wrestrict warning.
References: <https://github.com/dxx-rebirth/dxx-rebirth/issues/388>