Use gsl from the system, via pkg-config when available.

[kalibera]

This patch switches to using gsl from the system, when available via pkg-config or otherwise using hard-coded library dependencies. It makes the package work with gsl from Rtools42-45. Behavior with previous versions of R is not affected, as this uses the .ucrt version of Makevars.

This silences a warning (now on CRAN results) about using non-allowed external symbols.

[eddelbuettel]

Thank you so much -- I saw that on the CRAN results and was planning to ping you. I really appreciate that you got there first.

[eddelbuettel]

If you have a second to add a ChangeLog entry, I would as always appreciate one. Else I can carry it over from the RQuantLib fix.

[kalibera]

Sure, I added a ChangeLog entry.

[eddelbuettel]

Thanks. I may wait the polite week to avoid the nag on 'you just uploaded' and then refresh the package at CRAN. I should add an appropriate startup message that users may want to migrate to zigg which is just the PRNGs, and no comparison implementations -- which is what gets is the GSL tail here.