Add -iquote support

[Mike Stump <mrs at apple dot com>]

We need a way for a user to be able to use a conflicting header file in 
their project, as well as on the system and #include <> to get the 
system one, and #include "" to get their local one.  Normally -I- does 
this just fine, but, it has one small problem.  This disables looking 
in the containing directory for additional #includes.  In trivial 
projects that only have headers at the top level, one can put in a -I. 
at just the right spot and pick them up, but in less trivial software, 
putting in -Idir for all the places is, well, annoying.

So, the solutions that come to mind, would be to add a flag that allows 
one to not turn off searching in the local directory when -I- is used, 
or another flag to take the place of -I- that does what it does, but 
does not turn off searching in local directories, or add an option like 
-iquote, which would be the same as adding the directory to the end of 
the quote chain.

-iquote dir1 -iquote dir2

== -I dir1 -I dir2 -I- (without turning off searching in the containing 
directory)


Which option do people like best?  All of them are trivial enough to 
implement, as all the required semantics are in the existing code, just 
no way to select those semantics.

See split_quote_chain where we do quote_ignores_source_dir = true; for 
the semantic I want to avoid.

Another option, if people want this to be target dependent would be to 
add an -m option that sets pfile->quote_ignore_source_dir to false; 
though, I don't think I prefer that one.


The one we like best is -iquote, as it directly controls what we want 
to control.  It allows people to avoid -I- which is good as -I- makes 
the -I options more position dependent then is reasonable.

Ok?

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