Install dependencies on FreeBSD

For FreeBSD, there are prepackaged development binaries available. Install the gmt dependencies with:

# install necessary dependencies
sudo pkg install shells/bash devel/cmake ftp/curl science/netcdf

# install optional dependencies
sudo pkg install graphics/gdal devel/pcre math/fftw3-float math/lapack math/openblas print/ghostscript9

# to enable movie-making
sudo pkg install graphics/GraphicsMagick multimedia/ffmpeg

# to enable document viewing via gmt docs
sudo pkg install xdg-utils

# to enable testing
sudo pkg install graphics/GraphicsMagick

# to build the documentation
sudo pkg install py36-sphinx