[bug #47880] Allow updates to .INCLUDE_DIRS to change search path

[Paul D. Smith]

Follow-up Comment #4, bug #47880 (project make):

> 5. Introduce a command line switch to allow the user to remove the default
include directories. https://savannah.gnu.org/bugs/?60412.
I was hoping to avoid adding a new command line switch.  In general I don't
like them, plus we may need them for something else, plus having things on the
command line means that the behavior isn't encapsulated in the makefile itself
which is not great for many reasons.

So what I was hoping is that if this issue could be addressed then we wouldn't
need bug #60412.

> 2. Allow the makefile to modify the search path through adding -I<path> to
MAKEFLAGS. https://savannah.gnu.org/bugs/?58341.
This is, unfortunately, not good enough to do what you suggest here.  The
problem is that MAKEFLAGS is only parsed once before all makefiles are read,
and again after all makefiles are read.  It is not re-parsed every time it is
updated: see bug #45211 (enhancement).  That means it's unsuitable for
resetting -I since by the time it's reparsed all included files have already
been read in.

> Could be a problem. make tests use this feature. Users may use it as well.
Make tests _test_ this feature; obviously if it were changed the tests would
change to match the new behavior.  But users may be using it, I agree.  That's
especially true since up until now you couldn't learn the -I options by
examining MAKEFLAGS so looking at .INCLUDE_DIRS was the only way to see them. 
I can't speculate on how many peoples' makefiles care to examine the value of
the -I options or why they might do so.  I'm sure it's rare but I can't say no
one does it.

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