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.plugin.zsh

@@ -0,0 +1,72 @@
+# A collection of commands to make it easier to build projects.
+
+# Default `build` alias to select a `build-dir` then invoke a build, using an
+# alias means the configured build command's completion works out of the box.
+alias build="build-dir --build"
+
+# Detect installed debugger and set the `debug` alias to debug a program with
+# command line arguments.
+if [ `uname` = Linux ]; then
+  if which cgdb &> /dev/null; then
+    alias debug='cgdb --args'
+  elif which gdb &> /dev/null; then
+    alias debug='gdb --args'
+  fi
+elif [ `uname` = Darwin ]; then
+  which lldb &> /dev/null && \
+    alias debug='lldb --'
+fi
+
+# Store the path to `build-dir.py`, using `${0:a:h}` does not work in a
+# function because it will result in `pwd` not this scripts directory.
+_build_dir_py=${0:a:h}/build-dir.py
+
+# Interactively choose a `~build` directory for `build` to build.
+# TODO: Support arguments:
+# * [x] [-h,--help] - show this help message and exit
+# * [x] [--build] - execute the build commend
+# * [ ] <dir> - start in another directory, not `pwd`
+build-dir() {
+  # Get a unique filename to store the chosen build directory in.
+  [ `uname` = Darwin ] && local file=`mktemp` || local file=`tempfile`
+  # Prompt user to choose a build directory.
+  python $_build_dir_py $file;
+  local error=$?
+  # If the file exists, read the build directory from it, then delete it.
+  [ -f $file ] && build_dir=$PWD/`cat $file`; rm $file
+  # If choosing a build directory failed, return that error.
+  [[ "$error" = "0" ]] || return $error
+  # If `build.ninja` exists in alias `ninja`, return.
+  [ -f $build_dir/build.ninja ] && \
+    local build="ninja -C $build_dir"
+  # If `Makefile` exists in alias `make`, return.
+  if [ -f $build_dir/Makefile ]; then
+    [ `uname` = Darwin ] && \
+      local cpu_count=`sysctl -n hw.ncpu` ||
+      local cpu_count=`grep -c '^processor' /proc/cpuinfo`
+    local build="make -j $cpu_count -C $build_dir"
+  fi
+  # If the build variable is not defined the command could not be determined.
+  if [ -z $build ]; then
+    # TODO: Prompt user to choose a build command?
+    echo "could not determine build command"
+    return 1
+  fi
+  # Redefine the `build` alias and update the `~build` hash directory.
+  alias build="$build"
+  hash -d build=$build_dir
+  # If `--build` is specified then evaluate the command.
+  [[ "$1" = "--build" ]] && eval $build || true
+}
+
+# Build then run a target residing in `$build_dir/bin`.
+build-run() {
+  local target=$1; shift 1
+  build $target && $build_dir/bin/$target $*
+}
+
+# Build then debug a target residing in `$build_dir/bin`.
+build-debug() {
+  local target=$1; shift 1
+  build $target && debug $build_dir/bin/$target $*
+}