What is the best way to add a target specific system header directory

[Martin J. O'Riordan]

I am building GCC as a cross compiler for the Sparc Leon processor, it has
been working well for me for several years and is currently at v6.3.0.

However, a recent change I want to make is to provide an additional
non-standard system header directory to be searched when '<...>' is used,
and this directory is located relative to the 'gcc' binary; in this case
relative to:

    <wherever>/bin/triple-gcc

I have tried using '--with-native-system-header-dir' but this wants an
absolute path.  I want to avoid redefining 'INCLUDE_DEFAULTS' with my own
custom replacement.  Substituting for '/usr/include' is okay too, because
the host version is not meaningful the context of cross compilation.

Any recommendations on how to configure for this, or should I add something
(a '#define' for example) to 'gcc/gcc/config/sparc/sparc.h'?

Thanks,

    MartinO