The way the current standard is written, if I have the command line:
    c99 foo.c -I $HOME/include bar.c
and foo.c contains:
    #include "foo.h"
the foo.h header should not be found if it is in $HOME/include when
compiling foo.c even though it would be found in bar.c.  It should be
found in both foo.c and bar.c with the command line:
    c99 -I $HOME/include foo.c bar.c
Isn't this what is supposed to happen?

If the suggested change is made, you won't be able to have some headers
available for a subset of the source files being compiled.  This
probably doesn't happen frequently, but making this change might have
unwanted side effects.