Adding sudoless mode incorporating suggestions

This is a larger change but it:
1. Has a check to make sure all files in the Homebrew prefix are writable
2. Specifically handles CLT installation
3. Cleanly separates it out into its own function

Uses running `chmod` as a check. If it fails then something isn't writable.
Bonus is it also is a permissions repair.

For the CLT install with macOS sudoless mode, the first method will fail, so within the fallback we check for macOS_sudoless_mode and if we're using it then `abort` with the message about manual installation, instead of running the GUI pop up manual installation.

This still retains the original goal of installing in the official prefixes without sudo, so it doesn't result in an "Alternative (unsupported) installation".

Author: willmcginnis

install.sh

@@ -217,6 +217,18 @@ REQUIRED_GIT_VERSION=2.7.0   # HOMEBREW_MINIMUM_GIT_VERSION in brew.sh in Homebr
 export HOMEBREW_NO_ANALYTICS_THIS_RUN=1
 export HOMEBREW_NO_ANALYTICS_MESSAGE_OUTPUT=1
 
+macos_sudoless_mode() {
+  if [[ -n "${MACOS_SUDOLESS_SATISFIED-}" ]]
+  then
+    return "${MACOS_SUDOLESS_SATISFIED}"
+  fi
+ 
+  [[ -d "${HOMEBREW_PREFIX}" ]] && "${CHMOD[@]}" -R "ug+w" "${HOMEBREW_PREFIX}"
+  MACOS_SUDOLESS_SATISFIED=$?
+  
+  return "${MACOS_SUDOLESS_SATISFIED}"
+}
+
 unset HAVE_SUDO_ACCESS # unset this from the environment
 
 have_sudo_access() {
@@ -225,7 +237,7 @@ have_sudo_access() {
     return 1
   fi
 
-  if [[ -n "${HOMEBREW_ON_MACOS-}" ]] && [[ -w "${HOMEBREW_PREFIX}" ]]
+  if [[ -n "${HOMEBREW_ON_MACOS-}" ]] && macos_sudoless_mode
   then
     return 1
   fi
@@ -260,6 +272,7 @@ Or for a standard (non-admin) user, run:
 
 sudo mkdir -p "${HOMEBREW_PREFIX}"
 sudo chown -R "${USER}:${GROUP}" "${HOMEBREW_PREFIX}"
+sudo chmod -R "ug+w" "${HOMEBREW_PREFIX}"
 
 Then run this installer script again.
 EOABORT
@@ -877,11 +890,24 @@ fi
 # Headless install may have failed, so fallback to original 'xcode-select' method
 if should_install_command_line_tools && test -t 0
 then
-  ohai "Installing the Command Line Tools (expect a GUI popup):"
-  execute "/usr/bin/xcode-select" "--install"
-  echo "Press any key when the installation has completed."
-  getc
-  execute_sudo "/usr/bin/xcode-select" "--switch" "/Library/Developer/CommandLineTools"
+  if macos_sudoless_mode
+  then
+    abort "$(
+      cat <<EOABORT
+Installing without sudo on macOS requires manual Command Line Tools installation.  Run:
+
+  sudo xcode-select --install
+
+Then run this installer script again.
+EOABORT
+    )"
+  else
+    ohai "Installing the Command Line Tools (expect a GUI popup):"
+    execute "/usr/bin/xcode-select" "--install"
+    echo "Press any key when the installation has completed."
+    getc
+    execute_sudo "/usr/bin/xcode-select" "--switch" "/Library/Developer/CommandLineTools"
+  fi
 fi
 
 if [[ -n "${HOMEBREW_ON_MACOS-}" ]] && ! output="$(/usr/bin/xcrun clang 2>&1)" && [[ "${output}" == *"license"* ]]