From 2bfffb9f48a78de12faf5da77424c0cecb70d6eb Mon Sep 17 00:00:00 2001
From: Dan Willemsen <dwillemsen@google.com>
Date: Tue, 14 Jan 2020 10:56:53 -0800
Subject: [PATCH] Document the m4 prebuilts

Test: view in gitiles
Change-Id: I2defdb433313fb6744b09685d289e16d094cffdc
---
 Changes.md | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/Changes.md b/Changes.md
index 04e0161181..70e338cee6 100644
--- a/Changes.md
+++ b/Changes.md
@@ -1,5 +1,20 @@
 # Build System Changes for Android.mk Writers
 
+## `m4` is not available on `$PATH`
+
+There is a prebuilt of it available in prebuilts/build-tools, and a make
+variable `M4` that contains the path.
+
+Beyond the direct usage, whenever you use bison or flex directly, they call m4
+behind the scene, so you must set the M4 environment variable (and depend upon
+it for incremental build correctness):
+
+```
+$(intermediates)/foo.c: .KATI_IMPLICIT_OUTPUTS := $(intermediates)/foo.h
+$(intermediates)/foo.c: $(LOCAL_PATH)/foo.y $(M4) $(BISON) $(BISON_DATA)
+	M4=$(M4) $(BISON) ...
+```
+
 ## Rules executed within limited environment
 
 With `ALLOW_NINJA_ENV=false` (soon to be the default), ninja, and all the