[OE-core] gcc 6.1+ and isystem

[Khem Raj]

> On Aug 30, 2016, at 8:37 AM, Jack Mitchell <ml at embed.me.uk> wrote:
> 
> Some of the headers shipped with gcc 6.1 and above now use #include_next to try to and do clever things with munging system header files. Our injection of isystem into the build at 'meta/conf/bitbake.conf' seems to be causing some programs to fail to compile. A full explanation can be found at [1], a bug report from GCC specifying that it should only be used in extreme cases at [2].

you can say with isystem gcc let its users play smart things with its internal header search path order.

> 
> Since we seem to be adding -isystem unconditionally to BUILD_CFLAGS from bitbake, and that the default behavior has now changed should this be revisited? I'll admit that I am no where near experienced enough with GCC and friends internals to make a call on this one, I'm just looking for some input.
> 

Yes, I am aware of this fact and there has been a change to remove -isystem from BUILDSDK_CPPFLAGS, the problem with
BUILD_CPPFLAGS is different since it was added intentionally to override the system headers is in direct conflict with
what -isystem use is recommended for. If we were just complementing the default system includedirs it would be different
however. Should be not use -isystem by default systemwide ? may be. but we need to understand the effects
where, we now more or less build host packages against our own staged headers and link/run them using the hosts
libraries and this combination has been working however ugly it may look like. It also means we are using same headers
across all host distros which is good but then we run the host apps against the host libraries, causing another combination
more than often host systems have injected bugs into tools ( e.g. cross compilers ) which have shown to exhibit on target
very hard to trace issues like such have happened.

Can we then just act as a fallback to provide missing headers, after system headers, it falls into same problems or ordering
and while the header might be found in build sysroot, another header that this header needs may be needed from system

may be some tests by removing this from build options could be tried out, native packages like qt5 and  python3
should be tested since those definitely play their own games with headers.


> Regards,
> Jack.
> 
> [1] http://stackoverflow.com/questions/37218953/isystem-on-a-system-include-directory-causes-errors
> [2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70129

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