doc/README.SUSE: Minor content clean up (jsc#PED-5021)
    
    * Mark the user's build directory as a variable, not a command:
      'make -C $(your_build_dir)' -> 'make -C $YOUR_BUILD_DIR'.
    * Unify how to get the current directory: 'M=$(pwd)' -> 'M=$PWD'.
    * 'GIT' / 'git' -> 'Git'.