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
Type `SyncGLMethod` was defined conditional on DXX_USE_OGL, but header
`common/include/ogl_sync.h` used `SyncGLMethod` unconditionally. This
works fine in the normal build since SDL builds never include
`common/include/ogl_sync.h`, but broke the check_header_includes=1 test
since that compiles `common/include/ogl_sync.h` even for SDL builds.
Wrap type `ogl_sync` in `#if DXX_USE_OGL` to hide it from the
check_header_includes=1 test in SDL mode.