I get a similar error on the second line chnaged in the diff above:
ERROR! wget could not find http://releases.llvm.org/3.4/clang-3.4.src.tar.gz
http://llvm.org/releases/3.4/cfe-3.4.src.tar.gz, or could not write it to /home/of/src/blender-deps/CLANG-3.4.tar.gz, exiting

"clang-3.4.src.tar.gz" works, but "cfe-3.4.src.tar.gz" does not.
Note that the cfm-* file still has the "llvm.org/releases" in my case this is "http://llvm.org/releases/3.4/cfe-3.4.src.tar.gz"
The LLVM webserver redirects this to "http://releases.llvm.org/3.4/cfe-3.4.src.tar.gz", which does not exist either.
Google can't find this cfe-3.4.src.tar.gz file, except for a french FTP site.

I've been getting this same error intermittently for the last few days now, when trying to build blender 2.78c. Sometimes, the download works fine and the build proceeds as normal.

I see wget following a redirection to the new location as well, that seems to work (now, at least). When I try to download via web browser, I very often get 502 or 503 errors. I've asked on the llvm IRC channel, and apparently the server has been like this for some time now. The download for the latest version 5.0.1 seems to work reliably, but all older versions are flaky.

Would it be worth copying this dependency to a blender-controlled URL to ensure availability?

I don't get intermittent results, the linked file really does not exist. When looking at the manual download page: http://releases.llvm.org/download.html#3.4 I also noticed inconsistent file naming.
For 3.3, the "Clang source code" filename is cfe-3.3.src.tar.gz but for For 3.4, the "Clang source code" filename is clang-3.4.src.tar.gz and for 3.4.1 it's back to "cfe".