all: add static and release Makefile targets

BUILDING.md

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+# Building TinyGo
+
+TinyGo depends on LLVM and libclang, which are both big C++ libraries. There are
+two ways these can be linked: dynamically and statically. The default is dynamic
+linking because it is fast and works almost out of the box on Debian-based
+systems with the right libraries installed.
+
+This guide describes how to statically link TinyGo against LLVM and libclang so
+that the binary can be easily moved between systems.
+
+## Dependencies
+
+LLVM and Clang are both quite light on dependencies, requiring only standard
+build tools to be built. Go is of course necessary to build TinyGo itself.
+
+  * Go (1.11+)
+  * [dep](https://golang.github.io/dep/)
+  * Standard build tools (gcc/clang)
+  * git or subversion
+  * CMake
+  * [Ninja](https://ninja-build.org/) or make (preferably Ninja)
+
+The rest of this guide assumes you're running Linux, but it should be equivalent
+on a different system like Mac.
+
+## Download the source
+
+The first step is to get the source code. Place it in some directory, assuming
+`$HOME/src` here, but you can pick a different one of course:
+
+    git clone -b release_70 https://github.com/llvm-mirror/llvm.git $HOME/src/llvm
+    git clone -b release_70 https://github.com/llvm-mirror/clang.git $HOME/src/llvm/tools/clang
+    go get -d github.com/tinygo-org/tinygo
+    cd $HOME/go/src/github.com/tinygo-org/tinygo
+    dep ensure -vendor-only # download dependencies
+
+Note that Clang must be placed inside the tools subdirectory of LLVM to be
+automatically built with the rest of the system.
+
+## Build LLVM and Clang
+
+Building LLVM is quite easy compared to some other software packages. However,
+the default configuration is _not_ optimized for distribution. It is optimized
+for development, meaning that binaries produce accurate error messages at the
+cost of huge binaries and slow compiles.
+
+Before configuring, you may want to set the following environment variables to
+speed up the build. Most Linux distributions ship with GCC as the default
+compiler, but Clang is significantly faster and uses much less memory while
+producing binaries that are about as fast.
+
+    export CC=clang
+    export CXX=clang++
+
+Make a build directory. LLVM requires out-of-tree builds:
+
+    mkdir $HOME/src/llvm-build
+    cd $HOME/src/llvm-build
+
+Configure LLVM with CMake:
+
+    cmake -G Ninja ../llvm "-DLLVM_TARGETS_TO_BUILD=X86;ARM;AArch64" "-DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD=AVR;WebAssembly" -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=OFF -DLIBCLANG_BUILD_STATIC=ON
+
+You can also choose a different build system than Ninja, but Ninja is fast.
+
+There are various options you can tune here, but the options given above are
+preferable for releases. Here is what they do:
+
+  * `LLVM_TARGETS_TO_BUILD` and `LLVM_EXPERIMENTAL_TARGETS_TO_BUILD`: the
+    targets that are natively supported by the LLVM code generators. The targets
+    listed here are the ones supported by TinyGo. Note that LLVM is a cross
+    compiler by default, unlike some other compilers.
+  * `CMAKE_BUILD_TYPE`: the default is Debug, which produces large inefficient
+    binaries that are easy to debug. We want small and fast binaries.
+  * `LLVM_ENABLE_ASSERTIONS`: the default is ON, which greatly slows down LLVM
+    and is only really useful during development. Disable them here.
+  * `LIBCLANG_BUILD_STATIC`: unlike LLVM, libclang is built as a shared library
+    by default. We want a static library for easy distribution.
+
+Now build it:
+
+    ninja # or make, if you choose make in the previous step
+
+This can take over an hour depending on the speed of your system.
+
+## Build TinyGo
+
+Now that you have a working version of LLVM, build TinyGo using it. You need to
+specify the directories to the LLVM build directory and to the Clang source.
+
+    cd $HOME/go/src/github.com/tinygo-org/tinygo
+    make static LLVM_BUILDDIR=$HOME/src/llvm-build CLANG_SRC=$HOME/src/llvm/tools/clang
+
+## Verify TinyGo
+
+Try running TinyGo:
+
+    ./build/tinygo help
+
+Also, make sure the `tinygo` binary really is statically linked. Check this
+using `ldd`:
+
+    ldd ./build/tinygo
+
+The result should not contain libclang or libLLVM.