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Document the m4 prebuilts 

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Change-Id: I2defdb433313fb6744b09685d289e16d094cffdc
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Dan Willemsen 2020-01-14 10:56:53 -08:00
parent 997661002a
commit 2bfffb9f48
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# 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