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>
Merge branch 'stable'

Conflicts:
	Makefile
	src/gd.mk
	src/gdal.mk
	src/imagemagick.mk
	src/pcl.mk
	src/qt.mk