GMT is an open source collection of ~130 tools for manipulating
geographic and Cartesian data sets and producing PostScript
illustrations ranging from simple x-y plots via contour maps to
artificially illuminated surfaces and 3D perspective views.
Change cmake default behaviour to outsource.
Ports that build out of source now simply can use "USES=cmake"
instead of "USES=cmake:outsource". Ports that fail to build
out of source now need to specify "USES=cmake:insource".
I tried to only set insource where explictely needed.
Exp-run by: antoine