Add pkg-config support

[ralmond]

More generally, need to figure out how to download Netica from within the Makefile for RNetica.

https://pluto.coe.fsu.edu/bugzilla/show_bug.cgi?id=5

[ralmond]

This is working and tested under Linux and MacOS (with pkg-config installed through home brew).
Currently code for pkg-config is in the configure.win script, but not tested under Windows.

All three platforms will correctly download Netica API package from Norsys.com if not pkg-config netica, NETICA_HOME environmental variable or --with-netica arg set.