index: fix dead link

Closes #482.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
gc: fix download

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Add target download-only-* to ease developing

Signed-off-by: Timothy Gu <timothygu99@gmail.com>

Conflicts:
	Makefile
qt: Fix downloading

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
imagemagick: fix download

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
gd: Fix download

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
gdal: Fix download

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
hdf5: Fix download

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
opencv: Fix download

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
pcl: Fix download

Signed-off-by: Timothy Gu <timothygu99@gmail.com>