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From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] [PATCH] CPPFLAGS+= in Makefile.target
Date: Fri, 25 Jan 2008 14:39:57 +0000
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To: qemu-devel@nongnu.org
Cc: Ian Jackson <Ian.Jackson@eu.citrix.com>

> Saying CPPFLAGS+= is much more convenient if for any reason the
> external build environment would like to pass unusual CPPFLAGS.

No. This doesn't do what you thing it does.
The most common way of overriding these variables is to pass them on the 
commandline, i.e. "make CPPFLAGS=-blah". This overrides all assignments to 
that variable including +=.

Paul