Use quantlib from the system when available via pkg-config.

[kalibera]

This changes Makevars.win to use QuantLib on Windows from the system when available via pkg-config, otherwise falling back to what has been done before. This will allow using QuantLib from Rtools - it will be available there from the next update. The package passes its checks with QuantLib in the development version of Rtools on my system.

[eddelbuettel]

Fabulous! Thank you so much.

Two small asks:

• Can you add yourself to ChangeLog? Two spaces between entries in the header line, eight spaces to the '*'. Or C-x 4 a in the one true editor.
• Is the change 'r-oldrel safe' once we switch to R 4.5.0? [ Edit: I guess CI just showed me that ... ]

[kalibera]

Yes, it should work with older versions of R. I've tested myself with R 4.4.0. When QuantLib is not found via pkg-config, it falls back to what has been done before my change.

[eddelbuettel]

Beautiful. Really really appreciate this.