Add support for new #Include file delimiters, linked to search order

[duncanmac99]

The suggestion here is to add support for a custom set of delimiters, such as "#include [file]", or "#include (file)". The advantage is that the choice of delimiter can be used to tell the pre-processor which directory/ies to search first -- or search only.

By default, one sets up a list of possible delimiters, say " '()', '[]', '{}', '^^', '%%' ". These delimiters would only be enabled if the new option '+Ksym' ( where 'sym' is one of '([{^%' ) appears on the command line. This becomes significant when one relates to where the new option appears with regard to the '-I' option(s). If the '+Ksym' option appears just before a particular '-I' directory, searches for any file using that delimiter should start with ^that^ directory and proceed from there. However, if the user adds a '-Ksym' option referencing the ^same^ symbol after at least one more '-I' option has been specified, then search for files surrounded with "sym" should be confined to only that directory/ies.

For example, the command-line "... -I 'path1' +K '[' -I 'path2' -I 'path3' ..." would instruct the pre-processor to start searching for any file named as "#include [file]" at directory 'path2', then proceed down the list to the system directory/ies, then (optionally) cause a "wrap-around" to search the previous directories as well.

However, the command-line "... -I 'path1' +K '[' -I 'path2' -I 'path3' -K '[' -I 'path4' ..." would tell the pre-processor to search ^only^ the directories 'path2' and 'path3' for any file named as "#include [file]".

If the option '+Ksym' does not appear for a given 'sym', then any reference to a #include with the delimiter 'sym' should cause a warning message and the delimiters to be Ignored. Thus, with the command-lines above, a reference to "#include {file}" should cause a warning diagnostic. The files affected can then be searched for using the default "" delimiter rules. It is likely that only one pair of '+Ksym' ... '-Ksym' arguments should be allowed for any given 'sym', but that can be relaxed if it is desired. The presence of '-Ksym' without a preceding '+Ksym' should then be viewed as a Fatal Error, as would the appearance of '-Ksym' without at least one -I directory just before it (i.e. without at least one such -I argument between '+Ksym' and the following '-Ksym').

[duncanmac99]

In the existing system, it is legal to specify "#include file", with no delimiters at all. One can add the command-line option '-Kr' to forbid that; if that appeared on the command-line, a declaration in that form would cause a diagnostic (and the file would be Ignored, and not Included).

If this is done, then "wrapping around" the list of files probably should be the default. But one could add '-Kw' to prevent such wrap-around, or '+Kw' to enable it again. If '-Kw' appears before a '+Ksym', then inhibit wrap-around for that "sym", and vice-versa.