Use -isystem for including /usr/local/include

[AMDmi3]

Summary

This makes software with strict Werror settings not fail on system includes. Example is devel/pructl which fails with -Werror -Weverything in histedit.h after adding USES=libedit.

/usr/local/include/histedit.h:41:9: error: macro name is a reserved identifier [-Werror,-Wreserved-id-macro]
#define _HISTEDIT_H_
        ^

and

/usr/local/include/histedit.h:191:14: error: padding struct 'struct HistEvent' with 4 bytes to align 'str' [-Werror,-Wpadded]
        const char      *str;
                         ^

Which is even more important, this flag should fix cases when a library is conflicting with it's own headers from the system. E.g. when libfoo is built and it has -I/usr/local/include -I. (in that order), it may pick up /usr/local/include/foo.h from it's previous version and break/miscompile. -isystem works in such a way than even if the same path is included with -I, it's still treated as system, regardless of argument ordering, so this is a reliable solution.

[bapt]

This is a good improvement we should have done before.

Please open a PR with a link to that review so we can have an exp-run on this except if you are 100% sure to have tested all consumers of this USES.

[AMDmi3]

In D3618#75098, @bapt wrote:

Please open a PR with a link to that review so we can have an exp-run on this except if you are 100% sure to have tested all consumers of this USES.

It would definitely be safer to have an exp-run.
PR: ports/203101

[jbeich]

Nice! @bapt previously scared me off with "it would break any effort on crossbuilding".

[bapt]

In D3618#75826, @jbeich wrote:

Nice! @bapt previously scared me off with "it would break any effort on crossbuilding".

Yes but, since I fixed up my mind and figured out actually both can play together nicely! (sorry back in the time I could not make my mind in the right order)