[SV 60412] Allow -I- to throw out the current directory path

[SV 60412] Allow -I- to throw out the current directory path

Accept a "-" directory value to the -I option to clear the set of
directories to be searched up to that point, including the default
directories.

* NEWS: Announce the change.
* doc/make.texi (Summary of Options): Add documentation.
* src/read.c (construct_include_path): Check for '-' and if found,
clear the list of directories to be searched.
* tests/scripts/options/dash-I: Add tests for -I-.
* tests/scripts/variables/INCLUDE_DIRS: Add tests for -I-.