Correct way to remove compiler flag

[Stephen torri]

In a project which I particpate with there are certain compiler flags
that need to be removed from the CFLAGS because they cause problems with
the compiled code.

For example if with a imaginary compiler I had the flags:

CFLAGS="-a -b -c d"

For a large part of the project the CFLAGS are fine but a few files
cannot have '-c'.

How do I remove it safely without redefinine CFLAGS within the
Makefile.am file?

Stephen
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