x11-wm/afterstep-stable: unbreak with clang 4.0

../../libAfterStep/libAfterStep.a(colorscheme.o): In function `make_ascolor_scheme':
colorscheme.c:(.text+0x1e1b): undefined reference to `make_grad_argb'
colorscheme.c:(.text+0x1e40): undefined reference to `make_grad_argb'
colorscheme.c:(.text+0x1e65): undefined reference to `make_grad_argb'
colorscheme.c:(.text+0x1e89): undefined reference to `make_grad_argb'
colorscheme.c:(.text+0x1eb1): undefined reference to `make_grad_argb'
../../libAfterStep/libAfterStep.a(colorscheme.o):colorscheme.c:(.text+0x1ed1): more undefined references to `make_grad_argb' follow

Reported by: antoine (via exp-run)