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
As a macro, it always refers to the global grd_curcanv. This interferes
with converting canvas handling to be an argument. Expand GWIDTH so
that uses of grd_curcanv can be changed individually.