easier way to get clang 3.9

docs/linux_build.md

@@ -14,10 +14,9 @@ sudo apt-get install cmake
 Next you need a version of [CLang compiler](http://releases.llvm.org/3.9.0/tools/clang/docs/ReleaseNotes.html) that supports `-std=c++14`.  Version 3.9 or newer should work.   The following commands will get you clang 3.9:
 ````
 sudo apt-get update
-wget -O - http://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
-sudo apt-add-repository "deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial-3.9 main"
+sudo add-apt-repository ppa:ubuntu-toolchain-r/test
 sudo apt-get update
-sudo apt-get install clang-3.9 lldb-3.9
+sudo apt-get install clang-3.9 clang++-3.9
 sudo update-alternatives --install /usr/bin/clang clang /usr/bin/clang-3.9 60 --slave /usr/bin/clang++ clang++ /usr/bin/clang++-3.9
 ````