Add a help target to the root Makefile

This adds a help target to the Makefile in the repository root that
shows what variables are available to control the make/build behaviour.
It currently looks like

  $ make help
  Variables to control make/build behaviour:

    make V=1 ...           - Enables verbose output
    make D=1 ...           - Enables debug builds (-O0)
    make E=0 ...           - Disables -Werror

    Variables can be combined.

Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

auto/make

@@ -553,4 +553,13 @@ include $NXT_MAKEFILE
 clean:
 		rm -rf $NXT_BUILD_DIR *.dSYM Makefile
 
+.PHONY: help
+help:
+	@echo "Variables to control make/build behaviour:"
+	@echo
+	@echo "  make V=1 ...           - Enables verbose output"
+	@echo "  make D=1 ...           - Enables debug builds (-O0)"
+	@echo "  make E=0 ...           - Disables -Werror"
+	@echo
+	@echo "  Variables can be combined."
 END