I'll attach a patch to ghostscript-gpl. It doesn't link pdftoraster with the LDFLAGS variable, which lets portage throw a QA warning. Also, the patch changes the linking order so linking with --as-needed works.
I didn't add it myself as I didn't know if you wanna bump the whole ghostscript-patchset for it.
I have also submitted that patch upstream:
Huge thanks Hanno, fixed in ghostscript trunk and now in >=ghostscript-gpl-8.64-r1.