customcc: Fix the setup of the /usr/include path to use.

customcc: Fix the setup of the /usr/include path to use.

With the recent O_DIRECTORY addition, this apparent bug in the script was
exposed. Using -isysroot/-isystem doesn't seem to cut it for what we want
here (which is to completely override the /usr/include path).

I didn't verify in 100% detail, but all evidence looks like the reason is
that when -I is present on the command line, the paths of the headers
which are included by headers from this directory are not prefixed with
-isysroot's argument.

So for now, use -iprefix and -iwithbeforeprefix for /usr/include, which
seem to do what we want.

While here, add our copyright.

Reported-by: marino