[Buildroot] [PATCH 1/2] package/Makefile.in should grab HOST_DIR headers using -isystem instead of -I.

[Yann E. MORIN <yann.morin.1998@free.fr>]

David, All,

On 2016-07-25 15:52 -0400, David Raeman spake thusly:
> HOST_CFLAGS includes a search path for HOST_DIR/usr/include using -I.  When
> HOST_CFLAGS is used by a package, these flags are passed to the compiler ahead
> of flags passed by the package's internal make system.  If a package has a
> header file with the same name as a header file in HOST_DIR, this causes the
> toolchain to prefer the file from the system include directory because its -I
> appears first on the command line.  I believe conflicts should prefer the file
> provided by the package.  This can be accomplished by using -isystem, which is
> more appropriate then -I for system-level include paths.
[--SNIP--]

In fact, this does not work, and the patch was reverted (for background).

What we need is that gcc follows the following sequence when looking for
headers:

 1- search paths specified with -I by the package

 2- search our $(HOST_DIR)/usr/include

 3- search the host system /usr/include

However, I can't find anything in the gcc manpage that allows to add the
search path we need in (2).

All I can see is -idirafter, to add search paths after all other
non-system and system paths.

So, probably the following would not be very far from what we'd have to
do:

    -nostdinc -idirafter $(HOST_DIR)/usr/include -idirafter /usr/include

This tells gcc to not look at the system include path, so it is left
with only the paths specified by the package, and then as a fallback,
out $(HOST_DIR)/usr/include, and then as a last resort, the host-system
/usr/include dirs, since the last two are -idirafter:

    -idirafter dir
        Search dir for header files, but do it after all directories
        specified with -I and the standard system directories have
        been exhausted. dir is  treated as a system include directory.

Thoughts?

David, would you care to see if:

 1- it fixes your own problem?

 2- it fixes the python issues we've had with -isystem?

Regards,
Yann E. MORIN.

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