In other words, changes proposed are:
* replacing /ix86/linux-gnu/ with /ix86-linux-gnu/
* installing all headers in Headers/ix86-linux-gnu/gnu-gnu-gnu/xxx which
matches the Libraries/ix86-linux-gnu/gnu-gnu-gnu/xxx directory, and means
you don't need to install cpu/os specific headers in a separate directory.
* removing -Ixxx/ix86/linux-gnu/ from the command line.
The flattened case is unchanged, except potentially GSConfig.h, which
should at the moment go in Headers/ix86/linux-gnu/GSConfig.h (and which
would end up being top-level in the flattened case then!), would be moved
into Headers/GNUstepBase/.