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.
The `:Build [<target> ...]` command utilises the `$BUILD_DIR` variable set
by `:BuildDir {directory}` to invoke `ninja` or `make` based on the
existence of `$BUILD_DIR/build.ninja` or `$BUILD_DIR/Makefile`
respectively. The following commands are invoked in a new `:terminal`
window.
* `ninja -C $BUILD_DIR [<target> ...]` when `$BUILD_DIR/build.ninja` exists
* `make -C $BUILD_DIR [<target> ...]` when `$BUILD_DIR/Makefile` exists
In the case of `ninja`, completion for the targets supplied to the
`:Build` command is made available by the `build#targets()` function.
The list of targets returned by `ninja -C $BUILD_DIR -t targets` is
processed to generate the list of targets. Support for `make` target
completion is less straight forwards so has been omitted for now.
