Cygwin-specific "chsh" installation issue fixed.

Installation previously assumed the existence of a "chsh" command in the current
${PATH}. Since Cygwin does *NOT* provide this command, installation now tests
for the existence of this command before attempting to run it.

tools/install.sh

@@ -39,12 +39,17 @@ sed -i -e "/export PATH=/ c\\
 export PATH=\"$PATH\"
 " ~/.zshrc
 
-TEST_CURRENT_SHELL=$(expr "$SHELL" : '.*/\(.*\)')
-if [ "$TEST_CURRENT_SHELL" != "zsh" ]; then
+# If this user's login shell is not already "zsh", attempt to switch.
+if [ "$(expr "$SHELL" : '.*/\(.*\)')" != "zsh" ]; then
+  # If this platform provides a "chsh" command (not Cygwin), do it, man!
+  if hash chsh >/dev/null 2>&1; then
     echo "\033[0;34mTime to change your default shell to zsh!\033[0m"
     chsh -s $(grep /zsh$ /etc/shells | tail -1)
+  # Else, suggest the user do so manually.
+  else
+    echo "\033[0;34mPlease manually change your default shell to zsh!\033[0m"
+  fi
 fi
-unset TEST_CURRENT_SHELL
 
 echo "\033[0;32m"'         __                                     __   '"\033[0m"
 echo "\033[0;32m"'  ____  / /_     ____ ___  __  __   ____  _____/ /_  '"\033[0m"