Add build utility commands

* `build` is an alias which on first use invokes `build-dir --build` to
  select a build directory, reconfigure the `build` alias, and invoke
  the `build` alias on the selected build directory.
* `debug` is an alias to the installed system native debugger.
* `build-dir` is a function to select a build directory by reconfiguring
  the `build` alias, it detects `build.ninja` or `Makefile` in the build
  directory and selects the appropriate `ninja` or `make` command.
  Depends on the `build-dir.py` Python script which uses the `pick`
  package to interactively select the build directory.
* `build-run` is a function which builds the specified target then
  attempts to run it, making the assumption it resides in the `bin`
  subdirectory of the build directory.
* `build-debug` is a function which build the specified tared then
  attempts to debug it using the system native debugger, making the
  assumption it resides in the `bin` subdirectory of the build
  directory.

build/build-dir.py

@@ -16,7 +16,7 @@ from pick import Picker
 
 
 def main():
-    """Main entry point to build_dir command."""
+    """Main entry point to build-dir.py script."""
     parser = ArgumentParser()
     parser.add_argument('output')
     parser.add_argument('--default', action='store_true')

build/build.plugin.zsh

@@ -47,16 +47,15 @@ build-dir() {
     local build="make -j $cpu_count -C $build_dir"
   fi
   # If the build variable is not defined the command could not be determined.
-  echo "$build"
   if [ -z $build ]; then
     # TODO: Prompt user to choose a build command?
     echo "could not determine build command"
     return 1
   fi
-  # Define the new alias.
+  # Redefine the new alias.
   alias build="$build"
   # If `--build` is specified then evaluate the command.
-  [[ "$1" = "--build" ]] && eval $build
+  [[ "$1" = "--build" ]] && eval $build || true
 }
 
 # Build then run a target residing in `$build_dir/bin`.