Use `popup_menu()` in `build#dir()` when the `:BuildDir` command is uses
without an argument to specify a build directory. If there are no build
directories, print an error message. If there is a single build
directory, use it. If there are multiple build directories use
`popup_menu()` to prompt the user to select the desired build directory.
Update `build#targets()` and `build#run()` with comments and error
messages.
Kenneth Benzie (Benie)
aaa8acf5a2