configure.py: "ninja help" target

This adds a "help" build target, which prints out important build
targets. The printing is done in a separate shell script, becaue "ninja"
insists on print out the "command" before executing it, which makes the
help text unreadable.

configure.py

@@ -2023,6 +2023,9 @@ with open(buildfile_tmp, 'w') as f:
         rule extract_node_exporter
             command = tar -C build -xvpf {node_exporter_filename} && rm -rfv build/node_exporter && mv -v build/{node_exporter_dirname} build/node_exporter
         build $builddir/node_exporter: extract_node_exporter | always
+        rule print_help
+             command = ./scripts/build-help.sh
+        build help: print_help | always
         ''').format(**globals()))
 
 os.rename(buildfile_tmp, buildfile)

scripts/build-help.sh

@@ -0,0 +1,19 @@
+#!/bin/sh
+
+cat << EOF
+usage: ninja [options] [targets...]
+
+Run \`ninja -h\` for help on supported options.
+
+Build targets:
+  build             Build artifacts for all configured build modes.
+  <mode>-build      Build artifacts for a specific build mode.
+
+Test targets:
+  test              Run tests for all configured build modes.
+  <mode>-test       Run tests for a specific build mode.
+
+Packaging targets:
+  dist              Build distribution packages (.rpm, .deb) for all build modes.
+  <mode>-dist       Build distribution packages (.rpm, .deb) for a specific build mode.
+EOF