Merge "Document the m4 prebuilts" am: 474de20a36
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Change-Id: Ib725b317b169c7a4ef547b412c618e16088d15ad
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Changes.md
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# Build System Changes for Android.mk Writers
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## `m4` is not available on `$PATH`
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There is a prebuilt of it available in prebuilts/build-tools, and a make
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variable `M4` that contains the path.
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Beyond the direct usage, whenever you use bison or flex directly, they call m4
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behind the scene, so you must set the M4 environment variable (and depend upon
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it for incremental build correctness):
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```
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$(intermediates)/foo.c: .KATI_IMPLICIT_OUTPUTS := $(intermediates)/foo.h
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$(intermediates)/foo.c: $(LOCAL_PATH)/foo.y $(M4) $(BISON) $(BISON_DATA)
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M4=$(M4) $(BISON) ...
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```
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## Rules executed within limited environment
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With `ALLOW_NINJA_ENV=false` (soon to be the default), ninja, and all the
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