customize MacOS building and installation

Author: laris

README.md

@@ -25,15 +25,44 @@ First make sure you've installed all dependencies for building, as specified in
 the main Rust repository [here](https://github.com/rust-lang/rust/#building-from-source).
 Then use the following commands:
 
+### Unix/Linux/*nix
+
+``` bash
+# Grab the avr-rust sources
+git clone https://github.com/avr-rust/rust.git --recursive
+
+# Create a directory to place built files in
+mkdir build && cd build
+
+# Generate Makefile using settings suitable for an experimental compiler
+../rust/configure \
+  --enable-debug \
+  --disable-docs \
+  --enable-llvm-assertions \
+  --enable-debug-assertions \
+  --enable-optimize \
+  --enable-llvm-release-debuginfo \
+  --experimental-targets=AVR \
+  --prefix=/opt/avr-rust
+
+# Build the compiler, optionally install it to /opt/avr-rust
+make
+make install
+
+# Register the toolchain with rustup
+rustup toolchain link avr-toolchain $(realpath $(find . -name 'stage2'))
+
+# Optionally enable the avr toolchain globally
+rustup default avr-toolchain
+```
 ### Notes for macOS
 
 The conventional install directory for macOS is `/usr/local/avr-rust` rather than the Linux default `/opt/avr-rust`, so before the steps below, you must:
 
-1. Edit `build/config.toml` to set `prefix = '/usr/local/avr-rust'`.
-2. Ensure `/usr/local/avr-rust` exists and has the correct permissions: `sudo mkdir /usr/local/avr-rust && sudo chown $USER:admin /usr/local/avr-rust`
+1. ~~Edit `build/config.toml` to set `prefix = '/usr/local/avr-rust'`.~~
+2. Ensure `/usr/local/avr-rust` exists and has the correct permissions: `sudo mkdir /usr/local/avr-rust && sudo chown ${USER}:admin /usr/local/avr-rust`
 
 Finally, `realpath` isn't included by default on macOS but is included in GNU Coreutils, so you can either `brew install coreutils` so the `rustup toolchain...` step works properly, or just use the explicit path (`rustup toolchain link avr-toolchain /usr/local/avr-rust`).
-
 ``` bash
 # Grab the avr-rust sources
 git clone https://github.com/avr-rust/rust.git --recursive
@@ -50,14 +79,14 @@ mkdir build && cd build
   --enable-optimize \
   --enable-llvm-release-debuginfo \
   --experimental-targets=AVR \
-  --prefix=/opt/avr-rust
+  --prefix=/usr/local/avr-rust
 
-# Build the compiler, optionally install it to /opt/avr-rust
+# Build the compiler, install it to /usr/local/avr-rust
 make
 make install
 
 # Register the toolchain with rustup
-rustup toolchain link avr-toolchain $(realpath $(find . -name 'stage2'))
+rustup toolchain link avr-toolchain /usr/local/avr-rust
 
 # Optionally enable the avr toolchain globally
 rustup default avr-toolchain