From mboxrd@z Thu Jan  1 00:00:00 1970
From: Roger Pau Monne <roger.pau@citrix.com>
Subject: Re: [PATCH 3/3] autoconf: correctly parse *_INCLUDES
 and	*_LIB env vars
Date: Tue, 12 Jun 2012 13:02:26 +0100
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To: Ian Jackson <Ian.Jackson@eu.citrix.com>
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Ian Jackson wrote:
> Roger Pau Monne writes ("[Xen-devel] [PATCH 3/3] autoconf: correctly parse *_INCLUDES and *_LIB env vars"):
>> Parse those options correctly, since the "+=" operator is not valid.
>> Also added CPPFLAGS, so headers checks don't give strange results.
> ...
>>   CFLAGS="$PREPEND_CFLAGS $CFLAGS $APPEND_CFLAGS"
>> +CPPFLAGS="$CFLAGS"

Without this I get the following (harmless but noisy) error:

checking yajl/yajl_version.h usability... yes
checking yajl/yajl_version.h presence... no
configure: WARNING: yajl/yajl_version.h: accepted by the compiler, 
rejected by the preprocessor!
configure: WARNING: yajl/yajl_version.h: proceeding with the compiler's 
result

> Surely that can't be right.

 From http://www.edwardrosten.com/code/autoconf/:

"Just append stuff to CFLAGS (for the C compiler), CPPFLAGS (for the C 
preprocessor, C and C++ compilers), CXXFLAGS (for the C++ compiler) and 
LIBS (for the linker)."

It seems like the preprocessor check uses CPPFLAGS instead of CFLAGS, so 
we have to set both.