Merge "Document the m4 prebuilts"

# 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