update Ubuntu build instructions

INSTALL.md

@@ -27,22 +27,15 @@ To use some BPFtrace features, minimum kernel versions are required:
 - CMake
 - Flex
 - Bison
-- LLVM & Clang 5.0 development packages
+- LLVM & Clang 5.0 (or 6.0) development packages
 - LibElf
 
-For example, installing the requirements on Ubuntu:
-
-```
-apt-get update
-apt-get install -y bison cmake flex g++ git libclang-5.0-dev libelf-dev llvm-5.0-dev zlib1g-dev
-```
-
 ### Compilation
 
-See previous requirements.
+See previous requirements, and specific targets in the sections that follow (Ubuntu, Docker).
 
 ```
-git clone https://github.com/ajor/bpftrace
+git clone https://github.com/iovisor/bpftrace
 mkdir -p bpftrace/build
 cd bpftrace/build
 cmake -DCMAKE_BUILD_TYPE=Debug ../
@@ -53,6 +46,41 @@ By default bpftrace will be built as a dynamically linked executable. If a stati
 
 The latest versions of BCC and Google Test will be downloaded on each build. To speed up builds and only download their sources on the first run, use the CMake option `-DOFFLINE_BUILDS:BOOL=ON`.
 
+To test that the build works, you can try running the test suite, and a one-liner:
+
+```
+./tests/bpftrace_test
+./src/bpftrace -e 'kprobe:do_nanosleep { printf("sleep by %s\n", comm); }'
+```
+
+## Ubuntu
+
+The llvm/clang packages that are currently available for Ubuntu have an issue, so we'll use the ones from llvm.org for now. The build instructions are:
+
+```
+vi /etc/apt/sources.list
+---append---
+# from https://apt.llvm.org/:
+deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial main
+deb-src http://apt.llvm.org/xenial/ llvm-toolchain-xenial main
+# 5.0
+deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial-5.0 main
+deb-src http://apt.llvm.org/xenial/ llvm-toolchain-xenial-5.0 main
+# 6.0
+deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial-6.0 main
+deb-src http://apt.llvm.org/xenial/ llvm-toolchain-xenial-6.0 main
+---append---
+apt-get update
+apt-get install -y bison cmake flex g++ git libelf-dev zlib1g-dev libfl-dev
+apt-get install clang-5.0 libclang-5.0-dev libclang-common-5.0-dev libclang1-5.0 libllvm5.0 llvm-5.0 llvm-5.0-dev llvm-5.0-runtime
+git clone https://github.com/iovisor/bpftrace
+cd bpftrace
+mkdir build; cd build; cmake -DCMAKE_BUILD_TYPE=DEBUG ..
+make -j8
+```
+
+The bpftrace binary will be in src/bpftrace under the build directory.
+
 ## Using Docker
 
 Building inside a Docker container will produce a statically linked bpftrace executable.