Include file search path

[mdf at FreeBSD.org]

I thought I knew something about how the compiler looks for include
files, but now I think maybe I don't know much. :-)

So here's what I'm pondering.  When I build a library, like e.g. libc,
where do the include files get pulled from?  They can't (shouldn't) be
the ones in /usr/include, but I don't see a -nostdinc like for the
kernel.  There are -I directives in the Makefile for
-I${.CURDIR}/include -I${.CURDIR}/../../include, etc., but that won't
remove /usr/include from the search path.

I see in the gcc documentation that -I paths are searched before the
standards paths.  But isn't the lack of -nostdinc a bug (not just for
libc, but for any library in /usr/src/lib)?  It somewhat feels to me
that all of the libraries and binaries in the source distribution
should use -nostdinc and include only from the source distribution
itself.  This isn't always an issue, but for source upgrades it seems
crucial, and for a hacker it saves difficulties with having to install
headers before re-building.

Is that the intent, and it's not fully implemented?  How badly would
things break if -nostdinc was included in e.g. bsd.lib.mk? (This would
break non-base libraries, yes?  But as a thought experiment for the
base, how far off are we?)

Thanks,
matthew