Fix incompatibility with clang

LLVM 3.5 does not support -O4 anymore.  -O4 is now considered
to be equivalent to -O3.  The semantics of the old -O4 (full
optimization with lto) must now be specified as explicitly
-O3 -flto.

LLVM 3.5 includes the LLVM gold plugin and recent versions of
binutils include the gold linker.  Therefore the only tweak
necessary is to symlink /usr/bin/ld to point to gold.

On XUbuntu 14.10 there seems to be a bug in the LLVM 3.5
packages.  The names of the LLVM symlinks in /usr/bin have the
3.5 suffix in them.  They need to be manually renamed to have
version independent names.