config.mk: add FREETYPELIBS and FREETYPEINC

These variables make it simpler to change the paths to this for ports.
`pkg-config` is avoided because it sucks, in particular for cross-compilation.

A commented path for *BSD is added, the Xft includes are located at:
/usr/X11R6/include there.

Also already bump the version number to 4.6, a release will come
approximately in August.