Add :Build command to invoke ninja or make

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.

autoload/build.vim

@@ -18,9 +18,10 @@ function! build#targets(ArgLead, CmdLine, CursorPos) abort
 endfunction
 
 function! build#run(...) abort
+  let l:build_dir = substitute(fnamemodify($BUILD_DIR, ":~:."), '\/$', '', '')
   if filereadable($BUILD_DIR.'/build.ninja')
-    execute 'terminal ninja -C '.$BUILD_DIR.' '.join(a:000, ' ')
+    execute 'terminal ninja -C '.l:build_dir.' '.join(a:000, ' ')
   elseif filereadable($BUILD_DIR.'/Makefile')
-    execute 'terminal make -C '.$BUILD_DIR.' '.join(a:000, ' ')
+    execute 'terminal make -C '.l:build_dir.' '.join(a:000, ' ')
   endif
 endfunction