doc(make/msys2): update instructions

doc/make/msys2.md

@@ -19,9 +19,6 @@ It has a package management system, [pacman][pacman], which is used in Arch Linu
 Here are the commands to install all dependencies needed to compile Lean on your machine.
 
 ```bash
-# Update Packages
-pacman -Sy
-
 # Install gcc (4.9.1)
 pacman -S mingw-w64-x86_64-gcc
 
@@ -34,15 +31,22 @@ pacman -S mingw-w64-x86_64-python2 mingw-w64-x86_64-ninja mingw-w64-x86_64-cmake
 # Install git
 pacman -S git
 ```
-Make sure that you add `c:\msys64\mingw64\bin` into the `PATH` environment variable.
+
+Close the msys2 shell and add `c:\msys64\mingw64\bin` into the `PATH`
+environment variable.
+
+ - Windows 7/8: Control Panel > System and Security > System > Advanced
+   System Settings > Environment Variables... > Edit User variable
+   `Path`
 
 ## Build Lean
 
-We assume that you already check out Lean at `c:\lean` directory. In the [msys2] shell,
-execute the following commands.
+In the [msys2] shell, execute the following commands.
 
 ```bash
-cd /c/lean/
+cd /c/
+git clone https://github.com/leanprover/lean
+cd lean
 mkdir build && cd build
 cmake -D CMAKE_CXX_COMPILER=g++.exe -G Ninja ../src
 ninja
@@ -59,7 +63,9 @@ pacman -S mingw-w64-x86_64-boost
 In the [msys2] shell, execute the following commands.
 
 ```bash
-cd /c/lean/
+cd /c/
+git clone https://github.com/leanprover/lean
+cd lean
 mkdir build && cd build
 cmake -D CMAKE_CXX_COMPILER=g++.exe -D BOOST=ON -G Ninja ../src
 ninja