+# BUILD_CMD is a generic macro to build a given target in a given
+# subdirectory. The target must be given through the shell variable
+# `target' and the subdirectory to build in must be given through `dir'.
+# This macro shouldn't be used directly, use RECURSIVE_BUILD_CMD or
+# BUILD_ONE_CMD instead.
+#
+# BUILD_ONE_CMD is a macro to build a given target in a given
+# subdirectory if that subdirectory is part of $(DIRS). It requires
+# exactly the same shell variables as BUILD_CMD.
+#
+# RECURSIVE_BUILD_CMD is a macro to build a given target in all
+# subdirectories defined in $(DIRS). It requires that the target
+# is given through the shell variable `target'.