BUILD: Makefile: Implements the help target

Use the header of the Makefile to output the help target.
diff --git a/Makefile b/Makefile
index 587bc04..4e909e3 100644
--- a/Makefile
+++ b/Makefile
@@ -966,6 +966,9 @@
 INCLUDES = $(wildcard include/*/*.h ebtree/*.h)
 DEP = $(INCLUDES) .build_opts
 
+help:
+	$(Q)sed -ne "/^[^#]*$$/q;s/^#\(.*\)/\1/p" Makefile
+
 # Used only to force a rebuild if some build options change
 .build_opts: $(shell rm -f .build_opts.new; echo \'$(TARGET) $(BUILD_OPTIONS) $(VERBOSE_CFLAGS)\' > .build_opts.new; if cmp -s .build_opts .build_opts.new; then rm -f .build_opts.new; else mv -f .build_opts.new .build_opts; fi)