From 2bbe38c57c2498ef801f195f2cce41241bbfcba6 Mon Sep 17 00:00:00 2001
From: P. J. McDermott <pjm@nac.net>
Date: Mon, 08 Oct 2012 15:42:28 -0400
Subject: Document the build makefile.

---
diff --git a/specs/spf-2.0/buildsys.mdwn b/specs/spf-2.0/buildsys.mdwn
index 77f1eea..e99794f 100644
--- a/specs/spf-2.0/buildsys.mdwn
+++ b/specs/spf-2.0/buildsys.mdwn
@@ -1,7 +1,49 @@
 Build Makefile [build]
 ======================
 
-TODO
+A file named `build` directs the process of building and installing data files
+to be provided by binary packages.  It must be a makefile, and it must be
+executable by all users.
+
+The first line of this file must name a **make**(1) interpreter in a magic
+number sample as follows:
+
+    #! /usr/bin/make -f
+
+Any number of spaces and/or tabs may be used after the `#!` magic number and
+before the `-f` argument.
+
+The following targets are required:
+
+  * `build`
+
+    The `build` target should perform all the configuration and compilation
+    needed to produce files for installation into binary packages.
+
+    The `build` target should create a file named `build` in the build work area
+    to prevent configuration and compilation from being performed multiple
+    times.
+
+    The `build` target must not do anything that might require superuser
+    privilege.
+
+  * `install`
+
+    The `install` target should install data files into the data directories of
+    binary packages (see $[sectlink][work-area] for information on these
+    directories).
+
+    The `install` target should depend on the `build` target.
+
+    The `install` target shall be invoked as the superuser.
+
+The required targets and their dependencies must not require interactive user
+input.
+
+Additional targets may exist in the `build` makefile.
+
+Certain environment variables (see $[sectlink][env-vars]) shall be set when the
+`build` makefile is executed.
 
 
 Build Work Area [work-area]
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